[dosep] Run tests in a more parallel fashion

Summary:
Due to in-tree builds, we were parallelizing the tests at the directory
level. Now that the tests are built out-of-tree, we can remove this
limitation and paralelize at file level instead.

This decreases test suite time by about 10% for me, which is not
world-shattering, but it makes the code slightly simpler and will also
allow us to merge tests which were artificially spread over multiple
folders (TestConcurrentEvents...) to work-around this limitation.

To make this work, I've also needed to include the test file name in the
build directory name, as just the test method name is not unique enough
(plenty of tests have a test method called "test" or similar).

While doing this, I've found a couple of tests that are taking waaay longer then
they ought to (TestBreakpointCaseSensitivity -- 90 seconds), which I plan to
look into in the future.

Reviewers: aprantl

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D43335

llvm-svn: 325322