All tests based on APITestCase can be slow. I've also seen more than one
Jenkins failure due to GlobalTest::testMerge timing out.
Also, added a meta-test on APITestCase to make sure that all its
subclasses are marked with @group medium or @group large, to prevent new
tests from re-causing the bug.
Change-Id: I48630736a3d06574876fd1fa3d90899cfbc48012
Changeset Iec98e472 changed the behavior of action=parse&page=... when
passed a page that does not exist: previously, it would return a
"missingtitle" error instead of assuming an empty page. As some people
had been depending on this old behavior, restore the error checking.
Change-Id: I4c76ce458ceb01e233c6074cd9251879013ec143
