Automated integration tests for Groups member management API endpoints

[tsibley]

Comprehensive automated tests for these endpoints/functions would be quite helpful and reassuring.

It's possible now that we have a testing environment (#598, #656, #657), but there are some crucial test/development pieces still missing which we need to address:

• Obtaining authn tokens (e.g. logging in) in tests (or mechanisms to bypass authn in tests 😬 but would prefer not to).
• Test fixtures for groups and users, and how to deal with concurrency (e.g. multiple instances of the tests running at the same time). Most tractable way is likely to create groups and users at test start up with names that combine a meaningful part and a unique part, and then parameterize the tests by the names.

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