Add distinct aggregation command tests

[danielfrankcom]

This change adds tests for the distinct aggregation command.

There is some overlap here with #191 as some of the framework changes are needed by both. I've included them here so they should merge cleanly.

[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: 0.75 (mixed)

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (4333+37 LOC, 16 files); LLM: Adds new test coverage for the distinct aggregation command, which is a meaningful functional addition to the test suite.

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