The verification that junit4 and junit5 APIs are not mixed in test sources is broken

We were using srcFolder in our pom.xml and this got changed in https://github.com/SpoonLabs/spoon-maven-plugin/pull/68 (backward compat was broken...).

Which means that since that time (hard to know in what version https://github.com/SpoonLabs/spoon-maven-plugin/pull/68 was released, that's not very nice either Putting 11.10 for lack of a better version...), we're not testing it...

The very least that the spoon maven plugin should have done is to stop with an error when {srcFolder}} is used.