Write tests for __Private\Resolution\resolve_function()

[lexidor]

I had to use it in DontUseAsioJoinLinter and there were 0 known usages.
There are no test cases either.
I figured out the API with some guess work.

The documentation I wrote might not be 100% accurate and it is better to demonstrate the functions capabilities using a couple of tests.

[jjergus]

there were 0 known usages.

I think it's used from hh-apidoc, definition-finder, and/or user-documentation

But I agree, we should have tests too.