feat: clearly indicate which resolver we're using (#885)

See what we documented at https://github.com/ooni/spec/pull/257

Reference issue: https://github.com/ooni/probe/issues/2238

See also the related ooni/spec PR: https://github.com/ooni/spec/pull/257

See also https://github.com/ooni/probe/issues/2237

While there, bump webconnectivity@v0.5 version because this change
has an impact onto the generated data format.

The drop in coverage is unavoidable because we've written some
tests for `measurex` to ensure we deal with DNS resolvers and transport
names correctly depending on the splitting policy we use.

(However, `measurex` is only used for the `tor` experiment and, per
the step-by-step design document, new experiments should use
`measurexlite` instead, so this is hopefully fine(TM).)

While there, fix a broken integration test that does not run in `-short` mode.

internal/tutorial/netxlite/README.md

@@ -25,7 +25,7 @@ Index:
 
 - [chapter04](chapter04) shows how to establish QUIC sessions;
 
-- [chapter05](chapter05) is about the "system" DNS resolver;
+- [chapter05](chapter05) is about the "stdlib" DNS resolver;
 
 - [chapter06](chapter06) discusses custom DNS-over-UDP resolvers;