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

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

internal/engine/netx/mockablex/dialer.go

@@ -0,0 +1,23 @@
+package mockablex
+
+import (
+	"context"
+	"net"
+)
+
+// dialer is the interface we expect from a dialer
+type dialer interface {
+	DialContext(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// Dialer is a mockable Dialer.
+type Dialer struct {
+	MockDialContext func(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// DialContext implements Dialer.DialContext.
+func (d Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return d.MockDialContext(ctx, network, address)
+}
+
+var _ dialer = Dialer{}