[lit] Extract TestFormat base class, for future use.
[lit] Simplify --time-tests code.
[lit] Factor out a separate Test.Result() object.
[lit] Allow formats to return lit.Test.Result instances directly.
[lit] Lift XFAIL handling to core infrastructure.
[tests] Update fma3 check to work with Py3.
[lit] Fix a couple lingering Py3 compat issues in ProgressBar.