Add $listDatabases command tests

[danielfrankcom]

This change adds tests for $listDatabases. The intent here is not to test the functionality of the operator, but just the spec, meaning the parameters/config it accepts, and the results it produces. Verifying the actual database listing through creating/deleting them is considered out of scope for these tests.

As part of this change I defined common expectation formats which are reused by a bunch of tests, as they expect pretty much the same thing in most cases. As many tests are focused on parameter structure and content, the expected result is identical in a lot of places. This helps to reduce some duplication across the files.

Ref: #26

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

Applied: compatibility test, enhancement
Project fields suggested: Component test-coverage · Priority P2 · Effort XL · Status Needs Review
Confidence: 0.85 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (2857+25 LOC, 16 files); LLM: Adds new compatibility test cases for the $listDatabases command, expanding test coverage within the tests directory.

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