Add end to end testing

[davenice]

When making changes to the source code for the extension, it's very difficult to be confident in the quality and that nothing has been broken.

This issue covers using a GUI testing framework to add some GUI-driven tests.
These should have a mocked up backend using something like Wiremock so that it's possible to (for instance) expand the tree against a mocked up system. (So that the tests can run without external infrastructure requirements).

An initial few tests could be added as part of this issue before a follow-up is created.

[github-actions]

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