tests: enable import checker for tests/**.py files

Several known-bad files are excluded as they couldn't be trivially fixed.
In principle, we should fix them first, however, it would have more risk
to keep Py3k porting going without the test coverage.

Still contrib/**.py aren't covered, which needs another round.

tests/test-check-module-imports.t

@@ -141,6 +141,15 here that we should still endeavor to fi
 hidden by deduplication algorithm in the cycle detector, so fixing
 these may expose other cycles.
 
-  $ hg locate 'mercurial/**.py' 'hgext/**.py' | sed 's-\\-/-g' | python "$import_checker" -
+Known-bad files are excluded by -X as some of them would produce unstable
+outputs, which should be fixed later.
+
+  $ hg locate 'mercurial/**.py' 'hgext/**.py' 'tests/**.py' \
+  > -X tests/test-hgweb-auth.py \
+  > -X tests/hypothesishelpers.py \
+  > -X tests/test-ctxmanager.py \
+  > -X tests/test-lock.py \
+  > -X tests/test-verify-repo-operations.py \
+  > | sed 's-\\-/-g' | python "$import_checker" -
   Import cycle: hgext.largefiles.basestore -> hgext.largefiles.localstore -> hgext.largefiles.basestore
   [1]