ThreadSanitizer: thread leak thread.c:75 in isc_thread_create (in `lib/isc/tests`)

All libisc unittests compiled from c5b6f215 fail with:

WARNING: ThreadSanitizer: thread leak (pid=18175)
  Thread T1 (tid=4538707, finished) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x2a9cd)
    #1 isc_thread_create thread.c:75 (libisc.1503.dylib:x86_64+0x46d1c)
    #2 isc_test_begin isctest.c:91 (counter_test:x86_64+0x1000017b3)
    #3 _setup counter_test.c:37 (counter_test:x86_64+0x10000154a)
    #4 cmocka_run_one_test_or_fixture <null> (libcmocka.0.dylib:x86_64+0x5a9d)
    #5 start <null> (libdyld.dylib:x86_64+0x112e4)

  And 3 more similar thread leaks.

SUMMARY: ThreadSanitizer: thread leak thread.c:75 in isc_thread_create

on exit.