VUU52: Add E2E tests to CI

[pling-scottlogic]

Description

Adds a job to the Test stage of the CI/CD pipeline to run Cypress end-to-end tests whenever a PR is raised. Tests are set to run in Chrome and Edge until the Firefox bug is fixed (#30).

Change List

• Add a job to test-ui.yml to run Cypress E2E tests
• Fix screenshot test:
  • Change selector for save layout panel
  • Update SHELL_WITH_NEW_THEME_URL constant to match changes to the NewTheme example
  • Point Cypress config to the correct index HTML file location

Closes #52

[cfisher-scottlogic]

LGTM!

I think it could be worth exploring screenshots for failing tests (if this doesn't happen automatically?) So if a test fails, you get a screenshot of the page to see what's going on which helps debug things (how far did it get, what can the user see, etc). I don't think it's essential because we can just do that locally if this action fails, but if it's not a difficult thing to achieve it might be worth it.