[starnix][libfmq] Fix unit tests on Fuchsia

Similar to previous change I8040e94109e86fb08d82a347999d3f8452a8e02c,
update the tests to use the event word atomic that comes from
RegisteredFutex.

Also, skip a test that relies on procfs to count fds.

Bug: b/344007647
Change-Id: Icbae11eff94acc4d745e28dfceb81536f09d5e43
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/android.googlesource.com/platform/system/libfmq/+/1059084
Reviewed-by: Benjamin Lerman <qsr@google.com>