Fix missing enabledByDefault in echcheck (#1655)

Closes: ooni/probe#2807

## Checklist

- [ ] I have read the [contribution
guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [ ] reference issue for this pull request: <!-- add URL here -->
- [ ] if you changed anything related to how experiments work and you
need to reflect these changes in the ooni/spec repository, please link
to the related ooni/spec pull request: <!-- add URL here -->
- [ ] if you changed code inside an experiment, make sure you bump its
version number

<!-- Reminder: Location of the issue tracker:
https://github.com/ooni/probe -->

## Description

Please, insert here a more detailed description.

internal/registry/echcheck.go

@@ -18,9 +18,10 @@ func init() {
 					*config.(*echcheck.Config),
 				)
 			},
-			canonicalName: canonicalName,
-			config:        &echcheck.Config{},
-			inputPolicy:   model.InputOptional,
+			canonicalName:    canonicalName,
+			config:           &echcheck.Config{},
+			enabledByDefault: true,
+			inputPolicy:      model.InputOptional,
 		}
 	}
 }

internal/registry/factory_test.go

@@ -593,9 +593,7 @@ func TestNewFactory(t *testing.T) {
 			inputPolicy:      model.InputOrStaticDefault,
 		},
 		"echcheck": {
-			// Note: echcheck is not enabled by default because we just introduced it
-			// into 3.19.0-alpha, which makes it a relatively new experiment.
-			//enabledByDefault: false,
+			enabledByDefault: true,
 			inputPolicy: model.InputOptional,
 		},
 		"example": {