Test: extension test coverage

[connor4312]

Refs https://github.com/microsoft/vscode/issues/203563

• [ ] anyOS
• [ ] anyOS

Complexity: 4 Roles: Developer

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This iteration, we have added coverage support to VS Code test CLI. You will either want to have an extension already using the CLI, or update one to use the CLI as a prerequisite for verifying this. See the instructions in the link on how to do this.

Once do you, verification is actually pretty simple:

• Start off at an old version of the VS Code test CLI, 0.0.5 or below. Have the extension test runner installed in VS Code.
• Run one or more tests with coverage. Verify you get a notification telling you that you need to update your CLI to get coverage support.
• Update to the latest version of the @vscode/test-cli in your project.
• Run again. Verify coverage gets reported and appears correctly in the editor.
• Manually run vscode-test from the command line using --coverage, and see that it works there. Look at the couple of coverage related options in vscode-test --help and verify they make sense and work as expected.