Simplify ApiQueryRecentChangesIntegrationTest

Simplify ApiQueryRecentChangesIntegrationTest

This patch does two major things to the newly introduced
ApiQueryRecentChangesIntegrationTest:

* It rips the confusingly complicated assertArraySubsetsEqual appart.

* It removes all wikitext content. Note that no assertion is done on this
content (not even in the test that is about adding a category). The tests
run just fine with automatically generated content. I had to introduce the
$i counter to make sure consecutive edits are done with different content.
Otherwise the later edit is not stored.

Change-Id: Iae585be86f3fd9bff7d2b4b7b218d5b0b4258603