tests: cover event receiver with unit tests

[troggy]

What's in this PR

• added jest for testing
• created a test covering eventReceiver and exitChecker (partially)
• reworked eventReceiver to be a ES6 class, so implementation functions can benefit of a object state: rqms instance and queue names. Otherwise, I would need to pass these around to all the functions making function signatures bloated
• RQMS and queue names are now being passed as arguments and should be created by a caller. Otherwise, hard to mock, makes tests less easier to write and read