refactor(netx): move dialer's mockable types in mockablex (#368)

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

internal/engine/netx/mockablex/dialer_test.go

@@ -0,0 +1,25 @@
+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")
+	}
+}