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/encoder.go

@@ -0,0 +1,11 @@
+package mocks
+
+// Encoder allows mocking dnsx.Encoder.
+type Encoder struct {
+	MockEncode func(domain string, qtype uint16, padding bool) ([]byte, error)
+}
+
+// Encode calls MockEncode.
+func (e *Encoder) Encode(domain string, qtype uint16, padding bool) ([]byte, error) {
+	return e.MockEncode(domain, qtype, padding)
+}