integration testing via Solr Docker instances

[boonious]

Recent tests refactoring has removed the "live" tests that were tightly coupled to a specific Solr instance (#9). There is currently no integration testing in the codebase.

Tasks

• provide an off-the-shelf Solr test instance via Docker
• create a new integration test suite for basic Hui functions based on the instance
• provide custom Solr Docker instance for advanced test cases e.g. suggester, MLT
• update test suite for advanced integration test cases based on the custom instance