- Move temp directory name into local test config.

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

 - Finish documentation.

 - Optionally use multiprocessing.

 - Support llvmc and ocaml tests.

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

 - Provide test suite config for running unit tests.

 - Support a timeout / ulimit.

 - Support "disabling" tests? The advantage of making this distinct from XFAIL
   is it makes it more obvious that it is a temporary measure (and lit can put
   in a separate category).