fix(webconnectivity): pass TestHelpers to measurement (#736)

* Passed the TestHelpers field to RunAsyc and MeasureAsync. This reflects the test_helpers in the measurement.

* Spec already contains the correct output.

See  https://github.com/ooni/probe/issues/2073

Co-authored-by: decfox <decfox>

internal/model/experiment.go

@@ -39,6 +39,9 @@ type ExperimentAsyncTestKeys struct {
 	// MeasurementRuntime is the total measurement runtime.
 	MeasurementRuntime float64
 
+	// TestHelpers contains the test helpers used in the experiment
+	TestHelpers map[string]interface{}
+
 	// TestKeys contains the actual test keys.
 	TestKeys interface{}
 }