Add explain diagnostic tests

[PatersonProjects]

This PR adds the tests for the explain diagnostic command. Explain is an interesting case, as it has both more internal complexity and crosses more features than many of the other diagnostic commands. It also requires a collection as part of its own function call. Because of this, explain tests primarily rely on CommandTestCase rather than DiagnosticTestCase in order to take advantage of runtime collection resolution.

Ref: Issue #43, #438

[documentdb-triage-tool]

🤖 Auto-triaged by documentdb-triage-tool.

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

Reasoning

component from path globs (test-coverage); effort from diff stats (1061+0 LOC, 8 files); LLM: Adds new test coverage for the explain diagnostic command using CommandTestCase, touching the compatibility tests component within a single area.

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[eerxuan]

Please also verify queryPlanner.planCacheShapeHash and queryShapeHash exist.
We need a hash equivalence test for both planCacheShapeHash and queryShapeHash.
#438 (comment)

[PatersonProjects]

Please also verify queryPlanner.planCacheShapeHash and queryShapeHash exist. We need a hash equivalence test for both planCacheShapeHash and queryShapeHash. #438 (comment)

Added tests for both types of ShapeHash, covering cases in issue #438 and some further gaps. Please review the new file and let me know if I have missed anything.