SolrTestServer should maintain as little mutable state as possible - Remove QueryResponse field

[chriswk]

Currently SolrTestServer manipulates a class field for both SolrQuery and QueryResponse, these fields should ideally be immutable, and the methods currently manipulating them should instead return them for further usage.

Pros

• Query functions will be pure
• Easier to run tests in same class in parallell

  Cons

• Some of the current helper methods rather rely on this, and as such doing this might require a rewrite of tests already using the library.