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

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.

Differential Revision: https://reviews.llvm.org/D58527

llvm-svn: 355320