tests: sort imports in test-bookmarks-corner-case.t
test-check-module-imports.t breaks on py3 without this change.
Differential Revision: https://phab.mercurial-scm.org/D6701
fix: add some new test cases
These cover a couple of behaviors we were testing at Google that weren't
covered here before.
Differential Revision: https://phab.mercurial-scm.org/D6698