Implement idempotency test for run_key_migration_functions

[acwhite211]

Fixes #8042

Adds a Django regression test for the run_key_migration_functions management command to verify that it can be run repeatedly without creating duplicate migration-backed records.

The test snapshots relevant database record counts before the command runs, after the first run, and after a second run. The first run is expected to create or backfill records, while the second run must leave tracked record counts unchanged.

There is tracking on counts for models touched by the command, including collection object types, picklists, schema config records, permissions records, app resource dirs, uniqueness rules, and tectonic unit records. I also went ahead and added user-created picklist and permission records before the first run and between runs to simulate normal Specify 7 usage. We could try doing more here to simulate other Specify 7 usage.

Checklist

• Self-review the PR after opening it to make sure the changes look good and
  self-explanatory (or properly documented)
• Add relevant issue to release milestone
• Add pr to documentation list
• Add automated tests

Testing instructions

• See the the back-end testing github action runs successfully.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
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