Add unit tests for probes and checks.

[vrongmeal]

All the checks rely on various user inputs and can encounter many corner cases. Probes may encounter some race conditions as well. Writing tests is required for the same.

[fristonio]

While adding tests is good, I want to know where can we encounter race conditions in the probes?

[vrongmeal]

That I wrote in context with the websocket probe. The way i implemented it first had some issues but I changed it. Anyhow, yeah, we should write tests.