GH Actions: fail the build if a test run fails
GH Actions: fail the build if a test run fails

The way things were set up now in the `phpunit` job, no matter whether tests passed or failed, the workflow would always continue.

I suspect this may have been set-up this way to make sure that all variations of test runs will actually be run ?
The downside is that, while you will see a :x: for the individual build, the workflow will not be marked as failed if a test runs fails.

I'm proposing to change this now by:
* Removing the `continue-on-error` for the test run.
* Adding the `fail-fast` key and setting it to `false`.
    By default this key is set to `true`, which means that if any individual build within the job fails, all other builds within the job will be cancelled.
    By setting it to `false`, all builds in the matrix will still be run, but if any of them fail, the workflow will be marked as "failed".

phpDocumentor/ReflectionCommon