AuthManagerTest: Fix for PHPUnit 6

AuthManagerTest: Fix for PHPUnit 6

PHPUnit 6 apparently needs methods explicitly declared as mockable in
some cases where PHPUnit 4 didn't, specifically
'providerAllowsAuthenticationDataChange' in testGetAuthenticationRequests.

That also means we need to remove the (fortunately unnecessary) forcing
of the mocked class name in there, or else it reuses the generic
configuration created under that name for earlier tests.

For good measure, let's also remove the mocked class name forcing from
several other places where it isn't needed (but was presumably
copy-pasted from the places where it was).

Change-Id: If12ed836c7e47797f20deb0bdaa3ea852a921069