Clear DeferredUpdates state via setUp()

Clear DeferredUpdates state via setUp()

Best practices for phpunit say 'more setUp, less tearDown' which typically means
that tests are not responsible for cleaning up after themselves, rather tests
are in charge of setting the world up exactly as they expect it.  Part of this
expectation is that mediawiki does not maintain state between tests.

This patch clears DeferredUpdates from MediaWikiTestCase::setUp() because it has
internal global state.  This prevents state from leaking between test cases.

Bug: 72922
Change-Id: I57e6e89aa51c78bbf9d7f5151122b9e267d1a03e