UpdateTestController should allow using release history fixtures from arbitrary directories

Problem/Motivation

The Automatic Updates module → -- which will be moved into core → -- and some of its contrib sub-modules, needs to be able to use fake release history XML in its functional tests, since it deals with the update system.

Right now, we achieve this by wholesale copying UpdateTestController::updateTest() into the Automatic Updates module's testing framework. We will probably continue to need hacks like that...unless core could change UpdateTestController::updateTest() in such a way that it can serve release history XML from any arbitrary path.

Steps to reproduce

N/A

Proposed resolution

Change UpdateTestController::updateTest() so that it can serve fake release history XML from any path, not just the update_test module's fixtures.

Remaining tasks

Make it happen.

User interface changes

None.

API changes

None. This is part of a test module, but as far as I can tell, it's not part of the official testing system.

Data model changes

None.

Release notes snippet

None needed.