tests: Add helper function for ini_set with automatic cleanup

tests: Add helper function for ini_set with automatic cleanup

Some tests need to change the value of an ini setting, and typically implement
cleanup handling themselves, usually imperfectly.

Provide a helper function, $this->setIniSetting(), which will take care of
teardown in the same way that $this->setMwGlobals() does.

Change-Id: I7be4198592f0aaf73a28d3c60acb307a918b1a1f