refactor(tests): Enhance plugin testing with shared fixtures and beforeAll setup

[antonbsa]

What does this PR do?

This pull request introduces significant refactoring and enhancements to the test infrastructure for sample plugin tests. The changes focus on improving reusability, simplifying test setup, and centralizing shared utilities. Key updates include the creation of reusable fixtures, the removal of redundant code, and the addition of documentation for these utilities.

Test Infrastructure Refactoring

• Centralized Test Fixtures:

  • Added createSampleTest in tests/core/fixtures/sampleFixture.ts to generate a Playwright test instance with reusable sampleTest fixtures for plugin setup.
  • Added checkPluginAvailability in tests/core/fixtures/sampleBeforeAll.ts to handle plugin manifest fetching and environment variable setup in a shared beforeAll hook.

• Simplified Test Files:

  • Refactored samples/sample-action-button-dropdown-plugin/tests/test.spec.ts and samples/sample-actions-bar-plugin/tests/test.spec.ts to use the new shared fixtures, replacing custom setup logic.

Documentation Updates

• Added a new README.md in tests/core/fixtures to document the usage of createSampleTest and checkPluginAvailability, including examples and environment variable handling.
• Updated tests/README.md to reference the new documentation for setting up base spec files.

Configuration Updates

• Updated .env.template to include environment variables for custom plugin URLs (ACTION_BUTTON_DROPDOWN_URL, ACTIONS_BAR_URL) to support the new fixture setup.

[GuiLeme]

LGTM!