Split unit-tests and integration tests

[adamretter]

We should separate out tests into:

1. Unit Tests
2. Integration Tests
3. (optionally) Slow Integration tests

See https://github.com/eXist-db/exist/blob/develop/exist-core/pom.xml#L688

[duncdrum]

Once #2980 is done, we ll already have a running IT instance integrated and configured into the build workflow. We can then use this for other (xqsuite, java, cypress) int-tests. So 5.1.0 makes sense as a milestone.