Add $planCacheStats stage tests

[danielfrankcom]

This change adds test coverage for the $planCacheStats aggregation stage operator.

As part of this change I added target_collection and indexes to StageTestCase as I found I was needing them more than originally expected, and having to create too many custom StageTestCase derivations. For tests that don't need them they are optional, so this shouldn't cause any problems. They are also generally applicable, and while they may not be useful for many aggregation stage tests, they at least make sense still in the context of those stages.

As with tests like #166, I've used assertProperties to verify what I can about the expected output shape for this operator, but it is highly dependent on database engine internals. The main thing I focused on testing was the input/output spec, without trying to overreach and verify functionality of the operator.

Ref: #34

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

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

Reasoning

component from path globs (test-coverage, test-framework); effort from diff stats (979+5 LOC, 8 files); LLM: Adds new test coverage for the $planCacheStats aggregation stage, including StageTestCase extensions for target_collection and indexes fields across one component.

If a label is wrong, remove it manually and ping @patty-chow so the rules can be tuned. The bot will not re-label items that already have component labels.