Increase the number of available file descriptors

Increase the number of available file descriptors

notify_test and query_test run out of descriptors on some platforms.

    % ./notify_test
    [==========] Running 1 test(s).
    [ RUN      ] notify_start
    netmgr/tcp.c:369: REQUIRE(csock->fd >= 0) failed, back trace
    0   libisc-9.19.9-dev.dylib             0x00000001045c5080 default_callback + 72
    1   libisc-9.19.9-dev.dylib             0x00000001045c4ffc isc_assertion_failed + 56
    2   libisc-9.19.9-dev.dylib             0x00000001045b8bcc start_tcp_child + 304
    3   libisc-9.19.9-dev.dylib             0x00000001045b8710 isc_nm_listentcp + 636
    4   libisc-9.19.9-dev.dylib             0x00000001045b3e0c isc_nm_listenstreamdns + 344
    5   libns-9.19.9-dev.dylib              0x0000000104b90630 ns_interface_listentcp + 152
    6   libns-9.19.9-dev.dylib              0x0000000104b8f65c interface_setup + 488
    7   libns-9.19.9-dev.dylib              0x0000000104b8de98 do_scan + 2028
    8   libns-9.19.9-dev.dylib              0x0000000104b8d640 ns_interfacemgr_scan + 212
    9   notify_test                         0x000000010418ddd0 scan_interfaces + 44
    10  libisc-9.19.9-dev.dylib             0x00000001045d5fd8 isc__job_cb + 116
    11  libuv.1.dylib                       0x000000010545afe4 uv__run_idle + 152
    12  libuv.1.dylib                       0x0000000105455cd0 uv_run + 204
    13  libisc-9.19.9-dev.dylib             0x00000001045e1120 loop_run + 460
    14  libisc-9.19.9-dev.dylib             0x00000001045df824 loop_thread + 44
    15  libisc-9.19.9-dev.dylib             0x00000001045df6dc isc_loopmgr_run + 456
    16  notify_test                         0x000000010418b900 run_test_notify_start + 88
    17  libcmocka.0.dylib                   0x00000001054968c0 cmocka_run_one_test_or_fixture + 448
    18  libcmocka.0.dylib                   0x0000000105494ca4 _cmocka_run_group_tests + 848
    19  notify_test                         0x000000010418be60 main + 120
    20  libdyld.dylib                       0x0000000181509430 start + 4
    Abort
    % ./query_test
    [==========] Running 4 test(s).
    [ RUN      ] ns__query_sfcache
    netmgr/tcp.c:369: REQUIRE(csock->fd >= 0) failed, back trace
    0   libisc-9.19.9-dev.dylib             0x000000010294d080 default_callback + 72
    1   libisc-9.19.9-dev.dylib             0x000000010294cffc isc_assertion_failed + 56
    2   libisc-9.19.9-dev.dylib             0x0000000102940bcc start_tcp_child + 304
    3   libisc-9.19.9-dev.dylib             0x0000000102940710 isc_nm_listentcp + 636
    4   libisc-9.19.9-dev.dylib             0x000000010293be0c isc_nm_listenstreamdns + 344
    5   libns-9.19.9-dev.dylib              0x00000001026cc630 ns_interface_listentcp + 152
    6   libns-9.19.9-dev.dylib              0x00000001026cb65c interface_setup + 488
    7   libns-9.19.9-dev.dylib              0x00000001026c9e98 do_scan + 2028
    8   libns-9.19.9-dev.dylib              0x00000001026c9640 ns_interfacemgr_scan + 212
    9   query_test                          0x00000001026a1018 scan_interfaces + 44
    10  libisc-9.19.9-dev.dylib             0x000000010295dfd8 isc__job_cb + 116
    11  libuv.1.dylib                       0x0000000103996fe4 uv__run_idle + 152
    12  libuv.1.dylib                       0x0000000103991cd0 uv_run + 204
    13  libisc-9.19.9-dev.dylib             0x0000000102969120 loop_run + 460
    14  libisc-9.19.9-dev.dylib             0x0000000102967824 loop_thread + 44
    15  libisc-9.19.9-dev.dylib             0x00000001029676dc isc_loopmgr_run + 456
    16  query_test                          0x000000010269cf34 run_test_ns__query_sfcache + 88
    17  libcmocka.0.dylib                   0x00000001028068c0 cmocka_run_one_test_or_fixture + 448
    18  libcmocka.0.dylib                   0x0000000102804ca4 _cmocka_run_group_tests + 848
    19  query_test                          0x000000010269eab8 main + 116
    20  libdyld.dylib                       0x0000000181509430 start + 4
    Abort
    %

expected_creads can exceed the number of file descriptors on some
platforms.

    % ./udp_test
    [==========] Running 18 test(s).
    [ RUN      ] mock_listenudp_uv_udp_open
    [       OK ] mock_listenudp_uv_udp_open
    [ RUN      ] mock_listenudp_uv_udp_bind
    [       OK ] mock_listenudp_uv_udp_bind
    [ RUN      ] mock_listenudp_uv_udp_recv_start
    [       OK ] mock_listenudp_uv_udp_recv_start
    [ RUN      ] mock_udpconnect_uv_udp_open
    [       OK ] mock_udpconnect_uv_udp_open
    [ RUN      ] mock_udpconnect_uv_udp_bind
    [       OK ] mock_udpconnect_uv_udp_bind
    [ RUN      ] mock_udpconnect_uv_udp_connect
    [       OK ] mock_udpconnect_uv_udp_connect
    [ RUN      ] mock_udpconnect_uv_recv_buffer_size
    [       OK ] mock_udpconnect_uv_recv_buffer_size
    [ RUN      ] mock_udpconnect_uv_send_buffer_size
    [       OK ] mock_udpconnect_uv_send_buffer_size
    [ RUN      ] udp_noop
    [       OK ] udp_noop
    [ RUN      ] udp_noresponse
    [       OK ] udp_noresponse
    [ RUN      ] udp_shutdown_connect
    [       OK ] udp_shutdown_connect
    [ RUN      ] udp_shutdown_read
    [       OK ] udp_shutdown_read
    [ RUN      ] udp_cancel_read
    [       OK ] udp_cancel_read
    [ RUN      ] udp_timeout_recovery
    [       OK ] udp_timeout_recovery
    [ RUN      ] udp_double_read
    [       OK ] udp_double_read
    [ RUN      ] udp_recv_one
    [       OK ] udp_recv_one
    [ RUN      ] udp_recv_two
    [       OK ] udp_recv_two
    [ RUN      ] udp_recv_send
    udp__connect_cb(0x0, too many open files, 0x0)
    udp__connect_cb(0x0, too many open files, 0x0)
    udp__connect_cb(0x0, too many open files, 0x0)
    udp__connect_cb(0x0, too many open files, 0x0)
    udp__connect_cb(0x0, too many open files, 0x0)
    udp__connect_cb(0x0, too many open files, 0x0)
    %