manifest_xml: - Added doc and testing of unknown tags/elements.

Added this test to verify that older versions of repo can handle
"<superproject" element. Tested by adding "<iankaz" unknown element.

Tested the code with the following commands.

$ ./run_tests tests/test_manifest_xml.py
$ ./run_tests -v

Bug: https://crbug.com/gerrit/13709


Tested-by: Raman Tenneti <rtenneti@google.com>
Change-Id: I858d56f38cefcfcd14474efdd631a5a940c3ce47
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/293482


Reviewed-by: Mike Frysinger <vapier@google.com>