Add test coverage for per repository version uniqueness constraints

[pulpbot]

Author: @quba42 (quba42)

Redmine Issue: 7441, https://pulp.plan.io/issues/7441

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Provide test coverage for: https://pulp.plan.io/issues/6429

I deferred the test coverage in the interest of getting this issue merged, but I still think test coverage would be useful (and at this point I have a fairly detailed idea of what would be needed):

Minimal test:

Create a repository
Upload https://fixtures.pulpproject.org/debian/pool/asgard/o/odin/odin_1.0_ppc64.deb to it.
Also upload https://fixtures.pulpproject.org/debian-complex-dists/pool/asgard/o/odin/odin_1.0_ppc64.deb to it.
Assert that the second repository version has only one package in it, and that the first package was removed from the version.

If we want to be fancy:

Add both packages to an empty repository in a single API call.
Assert that the relevant error is thrown.

Add both packages to the repository from the minimal test using a single API call.
Assert that the new version still only has one package in it.

Extra fancy:

Build a third colliding package in the fixtures.
Add all three to the repository from the minimal test in a single API call.
Assert that the relevant error is thrown.

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