E2E: Added command and page to interact with app pages

[mckn]

What this PR does / why we need it:
This PR adds an AppPage that will make it easier to write tests agains pages added within a Grafana app plugin. It removes the need of constructing the base path /a/${pluginId} in every test. If we, for some reason, change the base path for apps we have this thin layer where we can cater for that in different versions of Grafana.

This functionality can be used in two different ways.

Either you call gotoAppPage({pluginId: '', path: ''}) to navigate to the page you want to test.

test('should navigate to app sub page when calling goto with path', async ({ gotoAppPage, page }) => {
  await gotoAppPage({ pluginId: 'redis-app', path: '/create' });
  await expect(page).toHaveURL(/.*\/a\/redis-app\/create$/);
});

Or you can create your own app fixture that you can use within your plugin to write tests.

// fixtures.ts
import { AppConfigPage, test as base } from '@grafana/plugin-e2e';
import pluginJson from '../src/plugin.json';

type AppTestFixture = { pageOne: AppPage };

export const test = base.extend<AppTestFixture>({
  pageOne: async ({ page, selectors, grafanaVersion, request }, use, testInfo) => {
    await use(
      new appPage(
        { page, selectors, grafanaVersion, request, testInfo },
        {
          pluginId: pluginJson.id,
          path: '/one',
        }
      )
    );
  },
});

export { expect } from '@grafana/plugin-e2e';

// test.spec.ts
test('navigating to page one', async ({ pageOne, page }) => {
  await pageOne.goto();
  await expect(page).toHaveURL(/.*\/a\/redis-app\/one$/)
});

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install @grafana/create-plugin@4.3.0-canary.821.518e194.0
npm install @grafana/plugin-e2e@0.22.0-canary.821.518e194.0
# or 
yarn add @grafana/create-plugin@4.3.0-canary.821.518e194.0
yarn add @grafana/plugin-e2e@0.22.0-canary.821.518e194.0

[github-actions]

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[sunker]

Looks great! Just one thing.

[grafana-plugins-platform-bot]

🚀 PR was released in @grafana/plugin-e2e@0.22.0 🚀