tst: Add test for sigtimedwait

tst: Add test for sigtimedwait

This change adds new test to assess sigtimedwait's timeout related
functionality - the sigset_t is configured for SIGUSR1, which will
not be triggered, so sigtimedwait just waits for timeout.

To be more specific - two use cases are checked:
- if sigtimedwait times out immediately when passed struct timespec has
  zero values of tv_nsec and tv_sec.
- if sigtimedwait times out after timeout specified in passed argument

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>