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
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