Document Component Testing changes in 8.5

[JReinhold]

Focused Tests

We can expand the section in https://storybook.js.org/docs/writing-tests/test-addon#storybook-ui
No code snippets necessary, it's all explainable with prose+video/images, similar to the existing content in that section.

Focused tests work on the story, component and group level.

Coverage is not enabled in focused tests. You can run a focused test while having watch mode enabled simultanously.

Coverage

• Coverage only accounts for stories, not any other Vitest tests configured. Using Vitest CLI instead includes the full coverage of the whole Vitest test suite, including stories.
• We report Statements as the overall project coverage metric
• We respect test.coverage.watermarks as a config for the pie chart color
• You need the Vitest dependencies
• Coverage and watch-mode are mutually exclusive. If coverage is enabled, enabling watch mode disables coverage.
• HTML report is available at localhost:6006/coverage/index.html.
• We respect most of the the user's settings at test.coverage (providers, etc), except for:

            enabled: true,
            clean: false,
            cleanOnRerun: !watchMode,
            reportOnFailure: true,
            reporter: [['html', {}], storybookCoverageReporter],
            reportsDirectory: resolvePathInStorybookCache(COVERAGE_DIRECTORY),

Misc

Configuration in UI

[JReinhold]

Also #29613