Clean up updates parser tests

[mxsasha]

The current tests in updates/tests/test_parser.py have one test for the change request processing and all the validators. This was not a bad idea at the time, but validators have become a lot more complex since, and the tests have become a bit of a mess. Everything to do with validator testing should be split out.

Important to keep in mind is that we keep sufficient integration test cover. Probably by still having some regular tests that cast a wide net to cover larger parts of authoritative change flow.