[#3366] Fixed failures in CfgIface UTs

[#3366] Fixed failures in CfgIface UTs

Tests were subject to wide variations in retry intervals
on certain VMs.  Reworked stop when done but allow for
longer overal timeout

src/lib/dhcp/testutils/pkt_filter_test_stub.cc
    PktFilterTestStub::openSocket() - close fd on simulated open error

src/lib/dhcpsrv/tests/cfg_iface_unittest.cc
    CfgIfaceTest::stopWait() - added func to stop io_service_

    CfgIfaceTest::TearDown() - close sockets before clearing ifaces

    TEST_F(CfgIfaceTest, retryOpenServiceSockets4)
    TEST_F(CfgIfaceTest, retryOpenServiceSockets4OmitBound)
    TEST_F(CfgIfaceTest, retryOpenServiceSockets6)
    TEST_F(CfgIfaceTest, retryOpenServiceSockets6OmitBound) - modified
    to use stopWait(), longer timeout, shorter wait