test: avoid regress hanging in macOS

test: avoid regress hanging in macOS

a backtrace of the process without this patch :
  * thread #1, queue = 'com.apple.main-thread', stop reason = signal SIGSTOP
    * frame #0: 0x00007fffb1fb7d96 libsystem_kernel.dylib`kevent + 10
      frame #1: 0x0000000108ed0a58 libevent-2.2.1.dylib`kq_dispatch + 696
      frame #2: 0x0000000108ec53d8 libevent-2.2.1.dylib`event_base_loop + 696
      frame #3: 0x0000000108e1763b regress`test_fork + 1931
      frame #4: 0x0000000108e5c7ad regress`run_legacy_test_fn + 45
      frame #5: 0x0000000108e690aa regress`testcase_run_one + 858
      frame #6: 0x0000000108e6954f regress`tinytest_main + 495
      frame #7: 0x0000000108e5c94b regress`main + 171
      frame #8: 0x00007fffb1e88235 libdyld.dylib`start + 1
      frame #9: 0x00007fffb1e88235 libdyld.dylib`start + 1

Issue: #546