Test setup improvement and more tests

[tahv0]

This module has very little tests or no tests at all for some scenarios. There is need for movement from "production tested" to improved test system.
I think that this module needs following tests

Module promises:

• Module promises reconnection and Error handling. no tests
• Tests for continuous health checks. no tests
• Proper error handling very little tests

Other tests could be:

• "Stress testing". Trigger hundreds or thousands events in Postgres db and handle events
• Test for module imports (commonjs, es6 import)
• listen many channels with single subscriber

[andywer]

The situation has improved a bit:
There is now an integration test that makes sure the library re-connects properly, that the channels are re-subscribed to and that notifications still work afterwards.

Test coverage should still be improved, though (see above).