Address data race over 'done' in zt_test

Address data race over 'done' in zt_test

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WARNING: ThreadSanitizer: data race (pid=697)
  Read of size 1 at 0x7ffcf51dd2b7 by main thread:
    #0 asyncload_zone /builds/isc-projects/bind9/lib/dns/tests/zt_test.c:206:2 (zt_test+0x4b835f)
    #1 <null> <null> (libcmocka.so.0+0x50d8)
    #2 __libc_start_main /build/glibc-vjB4T1/glibc-2.28/csu/../csu/libc-start.c:308:16 (libc.so.6+0x2409a)

  Previous write of size 1 at 0x7ffcf51dd2b7 by thread T15:
    #0 load_done /builds/isc-projects/bind9/lib/dns/tests/zt_test.c:89:8 (zt_test+0x4b8d99)
    #1 zone_asyncload /builds/isc-projects/bind9/lib/dns/zone.c:2243:3 (libdns.so.1110+0x1da0f6)
    #2 dispatch /builds/isc-projects/bind9/lib/isc/task.c:1157:7 (libisc.so.1107+0x50845)
    #3 run /builds/isc-projects/bind9/lib/isc/task.c:1331:2 (libisc.so.1107+0x4d799)