refactor(engine): allow scripts to register experiments (#860)

See https://github.com/ooni/probe/issues/2216

internal/registry/ndt.go

@@ -0,0 +1,23 @@
+package registry
+
+//
+// Registers the `ndt' experiment.
+//
+
+import (
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/ndt7"
+	"github.com/ooni/probe-cli/v3/internal/model"
+)
+
+func init() {
+	allexperiments["ndt"] = &Factory{
+		build: func(config interface{}) model.ExperimentMeasurer {
+			return ndt7.NewExperimentMeasurer(
+				*config.(*ndt7.Config),
+			)
+		},
+		config:        &ndt7.Config{},
+		interruptible: true,
+		inputPolicy:   model.InputNone,
+	}
+}