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/engine/experiment_integration_test.go

@@ -29,7 +29,18 @@ func TestCreateAll(t *testing.T) {
 		exp := builder.NewExperiment()
 		good := (exp.Name() == name)
 		if !good {
-			t.Fatal("unexpected experiment name")
+			// We have introduced the concept of versioned experiments in
+			// https://github.com/ooni/probe-cli/pull/882. This works like
+			// in brew: we append @vX.Y to the experiment name. So, here
+			// we're stripping the version specification and retry.
+			index := strings.Index(name, "@")
+			if index >= 0 {
+				name = name[:index]
+				if good := (exp.Name() == name); good {
+					continue
+				}
+			}
+			t.Fatal("unexpected experiment name", exp.Name(), name)
 		}
 	}
 }