Add $indexStats stage tests

[danielfrankcom]

This change adds tests for the $indexStats aggregation stage operator.

I defined a new IndexStatsTestCase object which extends StageTestCase. These tests require indexes and custom collections in some cases, and I didn't want to pollute the more widely used StageTestCase with these.

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 (1379+0 LOC, 8 files); LLM: Adds new test cases for the $indexStats aggregation stage, introducing a new test case class within the test-coverage component.

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