net: skip DNS tests on self-hosted Android

They were already skipped on tethered Android builders because
the tests are gated on GO_BUILDER_NAME being set and the Android
exec wrapper does not propagate GO_BUILDER_NAME.

Updates #31722

Change-Id: Ifd2c7daecc19a4e540d86d1f38083f43cc3e6b15
Reviewed-on: https://go-review.googlesource.com/c/go/+/175177
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/dial_test.go

@@ -973,11 +973,11 @@ func TestDialerControl(t *testing.T) {
 }
 
 // mustHaveExternalNetwork is like testenv.MustHaveExternalNetwork
-// except that it won't skip testing on non-iOS builders.
+// except that it won't skip testing on non-mobile builders.
 func mustHaveExternalNetwork(t *testing.T) {
 	t.Helper()
-	ios := runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64")
-	if testenv.Builder() == "" || ios {
+	mobile := runtime.GOOS == "android" || runtime.GOOS == "darwin" && (runtime.GOARCH == "arm" || runtime.GOARCH == "arm64")
+	if testenv.Builder() == "" || mobile {
 		testenv.MustHaveExternalNetwork(t)
 	}
 }