Allow for tests to be disabled at runtime

Summary:
The current implementation of the test suite allows the user to run
a certain subset of tests using '-p', but does not allow the inverse,
where a user wants to run all but some number of known failing tests.
Implement this functionality.

Reviewers: labath, zturner, tfiala

Subscribers: jingham, sas, lldb-commits

Differential Revision: https://reviews.llvm.org/D24629

llvm-svn: 282298