Provide UI Test documentation

[matthew-chirgwin]

Issue to write up and provide test documentation for Strimzi-ui.

This should cover:

• The types of testing required on areas of the codebase
  • I would suggest we aim for 100% code coverage as a minimum from day 0 - which should be checked/achieved by running tests on just the area in question
• Tools used for testing and why
  • Given piror discussion, would suggest Jest as a test runner/framework, React testing library
• How accesibility is tested, and accessible status maintained
• Updating existing documentation to show how/why these tools/approaches are used/how and where things should be tested

Initially this will cover the UI codebase only, but once we integrate the UI with Strimzi, it should also cover how these integrations are tested (eg via system test)