Investigate test flakiness

[MMMalik]

Are you reporting a feature request or a bug?

Task

Provide detailed reproduction steps (if any)

It happens occasionally that unit tests start failing without any apparent cause even if there were no code changes: https://github.com/ckeditor/ckeditor4-react/pull/292.

While they are called "unit tests", in fact these tests don't really fulfill the requirements of unit testing:

• There is a dependency on CDN
• Singleton CKEDITOR namespace is used across all tests and there is an unreliable teardown for editor instances
• Test cases are run sequentially
• Test cases are run in predefined order

Current stack:

• Karma is used because it allows to easily run tests in real browsers. Real browser is currently necessary due to fact that editor is loaded from CDN.
• @testing-library/react and related tools are used because they are pretty much staples when it comes to testing React code. In a typical scenario it's usually used with Jest and jsdom but we need a real browser here.
• Jasmine is used because it's a popular choice for Karma.

Perhaps our current approach can be improved?

Expected result

Unit tests are fast and reliable.

Actual result

Unit tests occasionally fail for unclear reasons.