Create embedded solr-cloud setup for unittest

[thomasegense]

We already have a embedded-solr setup(stand alone, single core) for unittest. And those unittest are of vital importance.

The new shard splitting that gives a substantial export speedup in multi sharded environments has no unittest that runs in the coded vs. a realistic solr-cloud setup.

Idea for a unittest.

1. Generate enough minimum sample documents (just required fields and content_text)
2. Index test-data into a single embedded solr. Maybe have a dedicated solr-master with no documents as this is the production setup.
3. index test.datra into an embedded solr cloud with 4 shards.
4. Validate output from solr is correct in both cases.