This diff addresses the following points of https://github.com/ooni/probe/issues/2135:
- [x] the `childResolver` type is useless and we can use `model.Resolver` directly;
- [x] we should use `model/mocks` instead of custom fakes;
- [x] we should not use `log.Log` rather we should use `model.DiscardLogger`;
- [x] make `timeLimitedLookup` easier to test with a `-short` tests;
- [x] ensure `timeLimitedLookup` returns as soon as its context expires regardless of the child resolver;
Subsequent diffs will address more points mentioned in there.
* ongoing
* while there, make sure we test everything
* reorganize previous commit
* ensure we have reasonable coverage in session
The code in here would be better with unit tests. We have too many
integration tests and the tests overall are too slow. But it's also
true that I should not write a giant diff as part of this PR.
* feat(sessionresolver): try many and use what works
* fix(sessionresolver): make sure we can use quic
* fix: the config struct is unnecessary
* fix: make kvstore optional
* feat: write simple integration test
* feat: start adding tests
* feat: continue writing tests
* fix(sessionresolver): add more unit tests
* fix(sessionresolver): finish adding tests
* refactor(sessionresolver): changes after code review
