[test] Add ability to expect timeouts

Summary:
This adds ability to mark test that do not complete due to hangs, crashes, etc., as "expected",
to avoid flagging the build red for a known problem. Functionally, this extends the scope of the
existing expectedFailureXXX decorators to cover these states as well. Once this is in, I will
start replacing the magic list of failing tests in dosep.py with our regular annotations which
should hopefully make code simpler.

Reviewers: tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D15530

llvm-svn: 255763