refactor: rename i/e/n/mockablex => i/netxmocks (#397)

Needed to more easily do https://github.com/ooni/probe/issues/1505

internal/engine/netx/mockablex/dialer_test.go

@@ -1,25 +0,0 @@
-package mockablex
-
-import (
-	"context"
-	"errors"
-	"net"
-	"testing"
-)
-
-func TestDialerWorks(t *testing.T) {
-	expected := errors.New("mocked error")
-	d := Dialer{
-		MockDialContext: func(ctx context.Context, network string, address string) (net.Conn, error) {
-			return nil, expected
-		},
-	}
-	ctx := context.Background()
-	conn, err := d.DialContext(ctx, "tcp", "8.8.8.8:53")
-	if !errors.Is(err, expected) {
-		t.Fatal("not the error we expected")
-	}
-	if conn != nil {
-		t.Fatal("expected nil conn")
-	}
-}