Define Cypress testing policy

This follows from https://github.com/corona-warn-app/cwa-website/issues/2476#issuecomment-1049661047.

The Cypress suite tests the web contents of https://www.coronawarn.app/ for errors as documented in the Testing section of the README document. The tests are not run regularly so issues may go undetected.

Tests can be run against Chrome (npm run test:chrome) or Firefox (npm run test:firefox) browsers. Chrome is the default browser selected by npm test.

The policy for testing should be defined.

• Should PR submitters check against Cypress before submitting changes?
• Should Cypress tests be run manually or automatically at regular intervals to catch errors?

[MikeMcC399]

Considering the issues which surfaced after releasing the FAQ updates last week, the Open Source Team might like to review their policy on testing with Cypress.

• The Cypress test suite did not cover and test the changes made to the web site (see #2612, #2613 & #2585)
• A significant number of links were broken by the web site update and these issues were not detected by Cypress (see for instance #2611 and several other issues opened since last Friday)
• The Cypress test suite failed on the update branch before the merge and then on the master branch after the merge

It would make sense to decide to rely more on Cypress testing to avoid unplanned temporary loss of functionality and unscheduled urgent work to fix issues not caught by formal testing.

[MikeMcC399]

• PR #2891 resolves this issue by adding the sentence:
  
  “To minimize the occurrence of errors we would ask you to perform all tests when contributing to our repository.”
  
  to README.md.