[MAINTENANCE] Parametrize test_cloud_backed_data_context_add_datasource

[anthonyburdi]

Changes proposed in this pull request:

• Parametrize and add helper methods and fixtures
• This PR is splitting up work into separate PRs from https://github.com/great-expectations/great_expectations/pull/5664 and https://github.com/great-expectations/great_expectations/pull/5763

Definition of Done

• [x] My code follows the Great Expectations style guide
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have run any local integration tests and made sure that nothing is broken.

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