[cmd/opampsupervisor] Add unit test suite

[evan-bradley]

Component(s)

cmd/opampsupervisor

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

There are currently no unit tests for the Supervisor. This poses a problem for maintaining the quality of the Supervisor and for quickly testing changes as they are made.

Describe the solution you'd like

Create a unit test suite for the Supervisor, ideally with something in the ballpark of 70% coverage.

Describe alternatives you've considered

No response

Additional context

No response

[bacherfl]

@evan-bradley I would like to work on this if this issue is still relevant (I suppose it is as the coverage could be improved still)