[lit] Small refactoring and cleanups in main.py

* Remove outdated precautions for Python versions < 2.7
* Remove dead code related to `maxIndividualTestTime` option
* Move printing of test and result summary out of main into its own
  function

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D68847

llvm-svn: 375046