[Bug] HTML Injection ignored by vitest plugin

[vanessayuenn]

Related to #29775

Users can define custom HTML files to inject scripts/style in Storybook's preview by adding a .storybook/preview-head.html or .storybook/preview-body.html file. This is automatically loaded in Storybook, but ignored in the Vitest plugin.

💡 Solutions/Action items:

• We can leverage the browser.testerHtmlPath to inject an HTML file into the test run, so that property could be automatically added internally via the Vitest plugin:

  // vitest.workspace.js
  export default defineWorkspace([{
      test: {
        browser: {
          testerHtmlPath: '.storybook/preview-head.html',
          // ...
        },
      }
  }])

This solution however only allows for a single file to be added. We could instead read the contents of these preview html files and leverage the [transformIndexHtml](https://vite.dev/guide/api-plugin#transformindexhtml) plugin hook as mentioned in the previous section to inject them into the test run.