(bug 43762) Mark slow unit test as @group medium

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
Merge "(bug 43762) Mark slow unit test as @group medium"