Reduce the test maintainance

[nulltoken]

Chore summary
From #1079 (comment)

From @nulltoken:

Just off-topic thought: This current test set up compels us to duplicate tests and use two different libs to intercept fetch calls and, depending on the the scope, use 3 different suffixes for test files (jest, karma or void).
This makes contributing and reviewing a little bit harder than it should be.
I feel there might be a better way to do this.
Thoughts?

From @P0lip

@P0lip Just off-topic thought: This current test set up compels us to duplicate tests and use two different libs to intercept fetch calls and, depending on the the scope, use 3 different suffixes for test files (jest, karma or void).

Yeah, that is true.

I feel there might be a better way to do this.

We could simply drop nock and use fetch-mock everywhere, I believe.
Nock is pretty cool though as it's an actual server.

Tasks

• Decrease reliance on built-in rulesets in tests - this makes it more challenging to update OAS (and possibly AsyncAPI) ruleset
• Use fetch-mock everywhere
• Find a way to avoid test duplication between node based and browser based testing (when dealing with window.fetch for instance)
• Remove test duplicates as much as possible