tests: add some commentary and diagnostics to test-run-tests.t
Hopefully this will give us a shot at fixing it on the Windows builder.
Differential Revision: https://phab.mercurial-scm.org/D1653
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.