(bug 44459) Ensure jqueryMsg treats plain messages as plain, with tests.

* Minor comment and naming fixes
* Consolidate parse and plain now that they do the same thing in mediawiki.js.
* Use jquerymsg-test- prefix to avoid any possible conflicts for real messages.

Change-Id: Ib8a01f114a061c6c7bc822b4507a6861712b0b98