lit: Incremental test scheduling

Add option -i to prioritize test runs by source file modification time and
previous failure state.

This optimal scheduling reduces typical test-and-fix iteration times to a
matter of seconds by rapidly answering the questions:

  1) Did my recent change fix tests that were previously failing?
  2) Do the tests I just wrote / modified still work?

The current implementation requires write permissions to the source tree
because it uses mtimes to track failures.