MediaWikiTestCase: Enforce children call parent::tearDown

Can't be implemented as a test case because that's not being
run before MediaWikiTestCase::tearDown.
Also there's no parent destructor, so no need to call one.

Also fixed BagOStuffTest.

Change-Id: Ifd8659b6c8a748c6716099f8822e2c50ab51bda7
Merge "MediaWikiTestCase: Enforce children call parent::tearDown"