tests: run many tests in $TESTTMP/repo instead of $TESTTMP
This is useful so we can store other files in $TESTTMP
(in particular tests that use docket files (nodemap, dirstate-v2) keep
file uids in $TESTTMP/UID)
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).