Users should be able to cancel a test in progress

Once a test is started from the AltUnity Tester window, users are not able to cancel/stop it.

This could save some waiting time in cases when tests were run by accident, or execution is taking too long.

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_{You can find the original issue from GitLab [here](https://gitlab.com/altom/altunity/altunitytester/-/issues/586).}