Folded grok processor into ingest-common module.
The rest tests have been moved to ingest-common module as well, because these tests don't run in the rest-api-spec module but in the distribution:integ-test-zip module
and adding a test plugin there felt just wrong to me. I think this is ok. I left a tiny ingest rest test behind in that tests with an empty pipeline.
Removed messy tests, these tests were already covered in the rest tests
Added ingest test plugin in test infra so that each module testing integration with ingest doesn't need write its own plugin
Moved reindex ingest tests to qa module
Closes#18490
Martijn van Groningen