Now that the test suite has very few expected failures remaining (2), it is a good

idea to also dump the session information for expected failures in addition to
failures and errors (tests with unexpected exceptions).

Modify the LLDBTestResult class which derives from unittest2.TextTestResult to
intercept the addExpectedFailure() method in order to mark the test instance as
needing its session information dumped.

llvm-svn: 118185