Re-enable some skipped tests on FreeBSD

These tests were skipped because they hung the old FreeBSD buildbot.
They pass (and do not hang) when run locally so enable them again.  We
will investigate further if they hang again once the new FreeBSD
buildbot is installed.

llvm.org/pr16684
llvm.org/pr18200
llvm.org/pr18230

llvm-svn: 248574