Added tests to check importing json feature

[IamEzio]

Related to #2895

This PR adds tests to check if a happy JSON (a valid consistent JSON) file is imported correctly.

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[IamEzio]

@dmos62 PTAL. Thanks!

[dmos62]

Approving. Note, the tests in mathesar/tests/imports/test_json.py are not API tests, thus how well they test full vertical intergration is not clear. I suspect you chose to use limited-verticality tests (for the lack of a better term) here, because that's what was done for CSVs. That's fine, but keep in mind that you probably don't want to keep investing in non-API tests: think of non-API tests as development aids, and think of API tests as correctness guarantees.