GH Actions: fail the build if a test run fails

[jrfnl]

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 in the workflow summary, the workflow will not be marked as failed, nor will the individual test builds be marked as failed.

If we look back at the last time the workflow was run completely, this can be seen in the annotations below the summary, which show that the Windows test runs all failed. https://github.com/phpDocumentor/ReflectionDocBlock/actions/runs/370378454

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".

Opening as draft to expose issue #287 properly.

[jaapio]

@jrfnl could you please update this pr?

[jrfnl]

@jaapio Rebased. Still in draft due to issue #287 needing solving first.

[jaapio]

@jrfnl #287 has been closed. I think this pr can be rebased and merged? Or do you want to introduce some more changes before merging this?