refactor(netx): move dns transports in netxlite/dnsx (#503)

While there, modernize the way in which we run tests to avoid
depending on the fake files scattered around the tree and to
use some well defined mock structures instead.

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

internal/netxlite/dnsx/mocks/roundtripper.go

@@ -0,0 +1,41 @@
+package mocks
+
+import "context"
+
+// RoundTripper allows mocking dnsx.RoundTripper.
+type RoundTripper struct {
+	MockRoundTrip func(ctx context.Context, query []byte) (reply []byte, err error)
+
+	MockRequiresPadding func() bool
+
+	MockNetwork func() string
+
+	MockAddress func() string
+
+	MockCloseIdleConnections func()
+}
+
+// RoundTrip calls MockRoundTrip.
+func (txp *RoundTripper) RoundTrip(ctx context.Context, query []byte) (reply []byte, err error) {
+	return txp.MockRoundTrip(ctx, query)
+}
+
+// RequiresPadding calls MockRequiresPadding.
+func (txp *RoundTripper) RequiresPadding() bool {
+	return txp.MockRequiresPadding()
+}
+
+// Network calls MockNetwork.
+func (txp *RoundTripper) Network() string {
+	return txp.MockNetwork()
+}
+
+// Address calls MockAddress.
+func (txp *RoundTripper) Address() string {
+	return txp.MockAddress()
+}
+
+// CloseIdleConnections calls MockCloseIdleConnections.
+func (txp *RoundTripper) CloseIdleConnections() {
+	txp.MockCloseIdleConnections()
+}