oonirun(v2.go): document and review implementation and tests

[bassosimone]

I am about to replace httpx with httpclientx (as implemented in ooni/probe-cli#1560) for the ./internal/oonirunv2 package. Before doing this, I would like to review the implementation and the tests to make sure we have adequate testing and that it's clear what we're doing and why we're doing it. The target files are v2.go and v2_test.go.

This work is part of more comprehensive work described by #2700.

[bassosimone]

It seems we're already covering these cases with tests, while using a real localhost HTTP server:

• common case where we fetch and execute a descriptor
• case where we fetch it but cannot update the cache
• case where we fetch it but there's no permission to automerge changes
• case where the server returns a literal "null"
• case where the server returns a descriptor with an empty test name

I do not think the job of the oonirun package would be to test its underlying HTTP library. That sad, because I am changing the underlying library, and for robustness, it won't certainly hurt to add:

• case where the connection is RST when attempting to fetch a descriptor
• case where the JSON does not parse

Since adding these new test cases would be quick, I think it makes sense since it adds a bit of robustness.