test: inject autoloader in Maintenance class

We used to inject the test autoloader from the global space after the
Maintenance class has been initialized.  That prevents us from using
tests classes (such as Mock) during the Maintenance setup.

This patch move the TestsAutoLoader.php require() at the beginning of
finalSetup() which would let use Mock classes.

Change-Id: Ia402eafae8407d1516e3d200ac97539e3681fdc6