[Test] allow tests to reuse a singel index across tests

This commit speeds up aggregations tests dramatically since they all
rely on the same index structure. Here we can safe a large amout of time
to not recreate the index for each small test.
Moved BackgroundIndexer to a top level class. Moved waitNumDocs to ElasticsearchIntegrationTestBase.

Update RelocationTests to use the above and unified testPrimaryRelocationWhileIndexing & testReplicaRelocationWhileIndexingRandom into a single randomized test.
[BUILD] Allow to set tests memory via the commandline