Create proper integration tests for ErrorProneChecksPlugin

[dmitrykuzmin]

Now, the ErrorProneChecksPlugin is tested with the help of several unit tests that test separate plugin parts.

After we migrated away from Mockito and lost access to creating testing mocks, some of these tests became really inefficient, and others now require to alternate the plugin code in order to implement the test.

All of this could be avoided if we had a proper ErrorProneChecksPluginTest which checked all main plugin usage scenarios, making the unit tests for separate parts unnecessary.