[tests] Import lit modules properly (via its package).

 - Lit itself ensures that its modules can be found, so this completely
   eliminates the need to modify PYTHONPATH.

 - This also makes it more obvious which parts of lit are being used in the
   custom format definition.


git-svn-id: https://llvm.org/svn/llvm-project/dragonegg/trunk@188003 91177308-0d34-0410-b5e6-96231b3b80d8