hcforwarder,healthcheck,cmd: Move grpc tests to its own packages

Move grpc tests to their own packages rather than using the cmd package.

This MR also refactors how prometheus and grpc servers are spawned to avoid having the same code copy&pasted every time that a grpc or a prometheus daemon is needed.

signal tests are dropped given that testing signal.NotifyContext is out of scope for this project. shandler.StlibSignalHandler and testutils.TestSignalHandler are provided to use signal.NotifyContext and allowing injecting custom ctx created for testing purposes respectively.