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.
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