test: Skip failure on FreeBSD from test infrastructure issue

test_convenience_registers_16bit_with_process_attach fails due to
pr18200.  The test has a @expectedFailureFreeBSD decorator, but it
appears this does not catch a RuntimError exception raised in the test
infrastructure, so the test still reports failure.  For now just skip
it.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@197174 91177308-0d34-0410-b5e6-96231b3b80d8