hooks: add some test about HGPLAIN setting and hooks In Mercurial 5.7, hooks are now ran with HGPLAIN set, which is a behavior change in. I could not find explicit test about it so I am adding one. The next changesets will introduce more change to help user mitigate the behavior change when needed. Differential Revision: https://phab.mercurial-scm.org/D9979