Write a how to test guide

[emmanuelbernard]

[stuartwdouglas]

I have updated our existing test documentation at #597.

I am not really sure what else can go in a dedicated testing guide at the moment though, as we don't really have support for much more. Until we add support for additional features the testing guide would pretty much just re-hash the contents on the getting started and native image guides.

[emmanuelbernard]

@stuartwdouglas ah but this is for the extension writing.
The idea was to get a Protean app testing guide, not extensions. We need to explain to user how they could test their app. And yes it's likely going to be simple at the beginning.

[emmanuelbernard]

@stuartwdouglas can you write a testing guide explaining @QuarkusTest, @SubstrateTest etc? The getting started guide has some core elements you can simply copy from.