[TST] Refactor tests

[alyssadai]

• Closes Write a test for behaviour of RETURN_AGG option #126
• Closes Refactor mock get method for testing to accept return object #266

Changes proposed in this pull request:

• Basic test added for structure of cohort query response when RETURN_AGG=true
• Turned mocked get function used across query tests into a few different fixtures, with variations based on the type of value/object to be returned by the mocked function
  • Specific return objects for the mocked function specified in test definitions via indirect parametrization
  • mock_successful_get (which depends on another fixture for the return object) kept a separate function to avoid overly complicated parametrization when writing tests (i.e., you cannot easily pass one fixture as a parameter to another fixture)

Checklist

• PR has an interpretable title with a prefix ([ENH], [FIX], [REF], [TST], [CI], [MNT], [INF], [MODEL], [DOC]) (see https://neurobagel.org/contributing/pull_requests for more info)
• PR has a label for the release changelog or skip-release (to be applied by maintainers only)
• PR links to GitHub issue with mention Closes #XXXX
• Tests pass
• Checks pass

For new features:

• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[coveralls]

Pull Request Test Coverage Report for Build 7702731794

• 0 of 52 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 99.784%

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Totals
Change from base Build 7648049836:	0.005%
Covered Lines:	925
Relevant Lines:	927

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💛 - Coveralls

[surchs]

Very cool PR @alyssadai, thanks for figuring all of this out.

Looking over the double parameterization here is impressive, but also a bit confusing:

1. when I read the parameterization I wonder: what is parameterized? There's only one parameter
2. when I read the test itself where I am using the mock I wonder: what does this mock actually do?

I think we could probably live with that, but my sense is that pytest offers a simpler way to achieve the same outcome: https://docs.pytest.org/en/6.2.x/fixture.html#using-markers-to-pass-data-to-fixtures

Maybe you could take a look at that section (and the "Factories as fixtures" just after it) and see if you agree.

Otherwise: 🧑‍🍳

[surchs]

🚀 PR was released in v0.2.0 🚀