[lldb] [test] Mark failing tests XFAIL on NetBSD

[mgorny]

Summary

Add a convenience 'expectedFailureNetBSD' decorator and mark all tests
currently failing on NetBSD with it. Also skip a few tests that hang
the test suite. This should establish a baseline for the test suite
and get us closer to enabling tests on buildbot. This will help us
catch regressions while we still have a lot of work to do to get tests
working.

It seems that there are also some flaky tests. I am going to address
them later on.

[mgorny]

Remove leftover decorators import.

[mgorny]

Update the list of failing tests to include tsan tests, and to remove some false positives. I'll probably update it once more soon enough but I'd still appreciate a comment if the style is ok.

[labath]

In general, feel free to add/remove the netbsd decorators as you see fit (or in consulation with kamil or whatever).

[krytarowski]

Feel free to commit this and next without review.

[mgorny]

Thanks. I'm going to retest/update it for master first.