test-extension: flush diagnostic message to stabilize chg output
Since chg server may create new file object for the attached stdout,
procutil.stdout is not ui.fout and the buffered procutil.stdout data wouldn't
be flushed at all. That's why test-extension.t passes without modification
on Python 2.
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).