Test: test coverage API finalization

[connor4312]

Refs: #123713

• anyOS @chrmarti
• anyOS @eleanorjboyd

Complexity: 4

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We have at long last finalized the test coverage API. Implementing an extension that produces data for the API from scratch is challenging, and requires domain-specific knowledge, so you might find playing with the selfhost test provider to be the most effective way to test it https://github.com/microsoft/vscode-selfhost-test-provider

The new types in the API include:

• TestRun.addCoverage
• FileCoverage
• TestRunProfile.loadDetailedCoverage
• FileCoverageDetail
• StatementCoverage
• DeclarationCoverage

Please:

• Do some testing that the API and UI functions as you would expect
• Read through the API and verify that its documentation makes sense to you
• See that UI interactions work nicely as you would expect (you can use any extension that produces coverage to test this part)