 - Move temp directory name into local test config.

 - Add --show-unsupported, don't show by default?

 - Optionally use multiprocessing.

 - Support valgrind in all configs, and LLVM style valgrind.

 - Support a timeout / ulimit.

 - Rename 'lit' injected variable for config to be lit_config.

 - Allow import of 'lit' in test suite definitions.

 - Create an explicit test suite object (instead of using the top-level
   TestingConfig object).

 - Allow 'lit' driver to cooperate with test suites to add options (or at least
   sanitize accepted params).

 - Consider move to identifying all tests by path-to-test-suite and then path to
   subtest, and don't use test suite names.

 - Consider move to change workflow to always load suites, then resolve command
   line arguments.

