refactor(netx/resolver): add CloseIdleConnections to RoundTripper (#501)

While there, also change to pointer receiver and use internal
testing for what are clearly unit tests.

Part of https://github.com/ooni/probe/issues/1591.

internal/engine/netx/resolver/fake_test.go

@@ -93,6 +93,10 @@ func (ft FakeTransport) Network() string {
 	return "fake"
 }
 
+func (fk FakeTransport) CloseIdleConnections() {
+	// nothing to do
+}
+
 type FakeEncoder struct {
 	Data []byte
 	Err  error