Prometheus: Improve integration tests with plugin-e2e

[bohandley]

What do we need to do?

• Move Prometheus selectors from e2e-selectors (here and here to versioned selectors in plugin-e2e
• Replace all selectors in core Prometheus and external Prometheus data sources
• Add a new folder with the plugin name in this directory
• Setup a new project that runs e2e tests here.

Why do we need to do this?

We previously used/currently use cypress e2e tests but are moving into using @grafana/plugin-e2e which uses Playwright. We can then improve integration tests, test across grafana versions, add this to github workflows in external data sources, and more.

Examples of tests using @grafana/plugin-e2e

• General help
• External Prometheus plugin-e2e with github workflow
• Infinity data source with plugin-e2e and github workflow