Problem: writing test suites for es4j apps is difficult

[yrashk]

Writing tests for es4j-powered apps is always a little bit of a hassle. To list a few issues:

• making sure every test gets the proper clean slate setup
• reducing the setup time
• making it compact and DRY
• (for some apps) a need to test against different journal and index engine backends.

Proposed solution: implement an eventsourcing-test module that will cover this.