tests: refactor manifest tests

The XmlManifestTests class is getting to be large and we're only
adding more to it.  Factor out the core logic into a new TestCase
so we can reuse it to better group more tests.

Change-Id: I5113444a4649a70ecfa8d83d3305959a953693f7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298222


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>