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/measurexlite/dns_test.go

@@ -301,8 +301,12 @@ func TestNewWrappedResolvers(t *testing.T) {
 		if resolvert.tx != trace {
 			t.Fatal("invalid trace")
 		}
-		if resolver.Network() != "system" {
-			t.Fatal("unexpected resolver network")
+		switch network := resolver.Network(); network {
+		case netxlite.StdlibResolverGetaddrinfo,
+			netxlite.StdlibResolverGolangNetResolver:
+		// ok
+		default:
+			t.Fatal("unexpected resolver network", network)
 		}
 	})
 }