Clarify where handlers are tested

[aeneasr]

Is your feature request related to a problem? Please describe.

Currently, handlers do not have any tests. I assume that these are done as part of the e2e tests. However, that makes it a bit difficult to understand what is tested where and how.

If tests are to remain in e2e, there should be at least a test stub in handler_test explaining where the test can be found. In my opinion, e2e tests do not replace proper integration testing and handlers should have their own tests also. They can re-use code if need be.