tests: use raw string in test-check-code.t
To avoid a SyntaxWarning on Python 3.8 due to invalid \ escape.
Differential Revision: https://phab.mercurial-scm.org/D5837
run-tests: set attributes in sorted order
Python 3.8 preserves insertion order in serialized output
(https://bugs.python.org/issue34160). Older Pythons serialized
in sorted order.
Let's make insertion order sorted so behavior is consistent across
Python versions.
Differential Revision: https://phab.mercurial-scm.org/D5831