xfail TestQueues.py and TestDarwinLogFilterMatchMessage.py

It looks like the message-content-retrieval aspect of DarwinLog
support is flaky, not just the regex match against it.  Slightly
less frequently than the regex matching, I am seeing the
direct string-match variant of log-message-content matching
also fail.

Tracked by:
llvm.org/pr30299
rdar://28237450

llvm-svn: 281251