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/decoder.go

@@ -0,0 +1,11 @@
+package mocks
+
+// Decoder allows mocking dnsx.Decoder.
+type Decoder struct {
+	MockDecode func(qtype uint16, reply []byte) ([]string, error)
+}
+
+// Decode calls MockDecode.
+func (e *Decoder) Decode(qtype uint16, reply []byte) ([]string, error) {
+	return e.MockDecode(qtype, reply)
+}