MultiTestRunner: Simplify, cleanup, and rename!

 - MultiTestRunner will eventually be renamed to 'lit', for LLVM integrated
   tester/testing. This has the pros of being pronouncable and short.

 - "Project" level configuration lives in 'lit.cfg', which is also what lit uses
   to find the root testing directory in some cases. This can be overridden for
   use in project files which want to precisely specify where things are.

 - TestRunner.py is not longer able to be invoked directly.
 
 - Moved some code to Util.py.

 - Introduced a configuration object.

 - Cleaned up --help, removed a few not-very-useful options.

 - Tried not to break anything that works. :)

llvm-svn: 77665