Refactor tests for better code reuse.

+ Create AbstractPluginIntegrationTest class for "plugin integration tests" and move all
  relevant code into it.
+ Refactor RelationshipGeneratorIntegrationTest to extend AbstractPluginIntegrationTest.
+ Update comments and test name in configuration test (extra bonus!)
Merge pull request #34 from leftclickben/RefactorIntegrationTestsAbstractBaseClassCodeReuse

Refactor tests for better code reuse.