This adds pre-built expected queries for:

Repository.objects.accessible()
Repository.objects.accessible_ids()

These functions live in reviewboard.scmtools.testing.queries, and can

be reused throughout our test suite. It's written in a verbose manner

that ensures we only ever test against finalized Q expressions, to

minimize chances of regressions or unexpected behavior.
The unit tests for .accessible() and .accessible_ids() have been

completely rewritten. They now check against these expected queries, but

they've also been expanded considerably to more thoroughly check access

with positive and negative test data, to better catch regressions. The

approach being used here will be carried over to other accessibility

tests for the next batch of this peformance work.

Unit tests pass.