fmq_unit_tests: Detect leaked FDs more efficiently

Instead of looping through the max number of allowed FDs +1, we now
check the number of FDs opened by the test process before and after
creating/destroying a number of queues.

Test: atest fmq_unit_tests
Bug: 343829026
Change-Id: I9fe29e418b2acafd3a12b1add96ea412e3eaa120
1 file changed