shm tests: Append PID to names passed to shm_open [BZ #26737]

Append PID to names passed to shm_open in shm tests to avoid random

FAIL: rt/tst-shm-cancel
FAIL: rt/tst-shm

due to the same name passed to shm_open and shm_unlink when more than
one "make check" running in parallel on the same machine.

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