run-tests: extract sorting of tests to own function TestRunner._run() is a large function and is difficult to follow. Let's extract the test sorting to its own function to make it shorter. When I refactored run-tests.py several years ago, I put a lot of functionality in methods. The prevailing Mercurial style is to use functions - not classes - where possible. While refactoring the code, I decided to undo this historical mistake of mine by moving the code to a standalone function. Differential Revision: https://phab.mercurial-scm.org/D1750