Improvements to testing blacklist

Summary:
This patch is necessary because individual test cases are not required
to have unique names. Therefore, test cases must now
be specified explicitly in the form <TestCase>.<TestMethod>.
Because it works by regex matching, passing just <TestCase> will
still disable an entire file.

This also allows for multiple exclusion files to be specified.

Reviewers: zturner, labath, jingham, tfiala

Subscribers: lldb-commits, sas

Differential Revision: https://reviews.llvm.org/D24988

llvm-svn: 283238