Blueprint planner: clean up pending MGS update test helpers (#8836)

This is mostly because I found the existing support stuff pretty
unwieldy when I went to try to extend it to do host OS updates too.

The changes here are _mostly_ refactoring. I did add one extra
assertion: we had a couple tests that would build a set of
`expected_updates`, then confirm all the updates produced by planning
were present in `expected_updates`. But they weren't confirming that
we'd seen _all_ the expected updates. When I added this assertion, the
`test_whole_system_simultaneous()` test failed. I think this is because
we can't actually update the whole system simultaneously: we do at most
one kind of update to a given board at once. I reworked this test some
to "update all the RoTs" then "update all the SPs" instead.

Author: jgallagher