Revert "Disable flaky test lib.libc.sys.setrlimit_test.setrlimit_stack"

The stack gap implementation is disabled by default now, so the test
passes.

This reverts commit dad71022bd7a8f95ab2ba656bec61e2424a1c3c5.

PR: 259969
MFC after: 1 week
Sponsored by: The FreeBSD Foundation