Establish unittest utils for galaxy-{data,objectstore} packages.

[jmchilton]

Progress toward #8364. Cleans up dependencies and properly packages things to allow:

• Removing symlink hacks for objectstore test helpers that allowed unit tests not properly packaged to be run with packages in objectstore and data packages.
• We can move test_model_discovery.py into galaxy-data's unit tests since everything is properly packaged.
• We can move test_model_store.py into galaxy-data's unit tests since everything is properly packaged - though I did some more work here to remove other dependencies as well.
• I moved test/unit/model and test/unit/dataset_collection into test/unit/data so they are included in galaxy-data's unit test suite.

In addition to packaging more tests and cleaning up hacks, this makes it a little more clear what the interface of Galaxy's app and config that is consumed by galaxy-data functionality (e.g. model stores).

How to test the changes?

• This is a refactoring of components with existing test coverage.

License

• I agree to license these contributions under Galaxy's current license.
• I agree to allow the Galaxy committers to license these and all my past contributions to the core galaxy codebase under the MIT license. If this condition is an issue, uncheck and just let us know why with an e-mail to galaxy-committers@lists.galaxyproject.org.

[jdavcs]

A very enthusiastic +1 for minimal galaxy mock!