DWIMy filterspecs for dotest.py

Summary:
dotest.py currently requires a filterspec to be of the
form `TestCase.test_method`.   This patch makes it more
flexible, so you can pass `TestModule.TestCase.test_method`
or `TestModule.TestCase` or `TestCase.test_method` or just
`test_method`.

This makes it more convenient to just copy a test name
out of the terminal after running a bunch of tests and use
it as a filterspec.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: JDevlieghere

Subscribers: jingham, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68545

llvm-svn: 373997