Enable StopHookForMultipleThreadsTestCase::test_stop_hook_multiple_threads_with_dwarf on FreeBSD

This test passes locally but was disabled due to pexpect issues on the
FreeBSD buildbot. That buildbot has been retired as it was overloaded,
and we will investigate again if this fails once a new buildbot is in
place. Noted by John Wolfe.

llvm.org/pr22784

llvm-svn: 247013