Refactor assertEqualXML into a testtools matcher

Not all tests inheriting from TestCase will need to check XML equality.
Moving this functionality to a matcher leaves it up to the test class to
decided whether or not it needs it.

Change-Id: Ib28ec3b5dd96f662ce0cd90c650434b24c63ad6c
Related-Bug: #1226466
Merge "Refactor assertEqualXML into a testtools matcher"