Deflake semaphore_test.
Deflake semaphore_test.

- Disable saving in tests that wait for EINTR.

- Do not execute async-signal-unsafe code after fork() (see fork(2)'s manpage,
  "After a fork in a multithreaded program ...")

- Check for errors returned by semctl(GETZCNT).

PiperOrigin-RevId: 340901353

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor