[test] Fix category-based skipping

Summary:
LLDBTestResult.hardMarkAsSkipped marked the whole class as skipped when the first class in the
test failed the category check. This meant that subsequent tests in the same class did not run
even if they were passing the category filter. Fix that.

Reviewers: tfiala

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D22213

llvm-svn: 275173