chore: improve testing and increase coverage (#794)

This diff improves testing and increases coverage inside the
./internal/netxlite and ./internal/tracex packages.

See https://github.com/ooni/probe/issues/2121

internal/netxlite/resolverserial.go

@@ -33,6 +33,8 @@ type SerialResolver struct {
 	Txp model.DNSTransport
 }
 
+var _ model.Resolver = &SerialResolver{}
+
 // NewUnwrappedSerialResolver creates a new, and unwrapped, SerialResolver instance.
 func NewUnwrappedSerialResolver(t model.DNSTransport) *SerialResolver {
 	return &SerialResolver{