Alerting: Fix stale values associated with states that have gone to NoData, unify values calculation (#89807)

* Unify values

* Fix with latest changes on main

* Fix up NaN test

* Keep refIDs with -1 as value

* Test that refIDs are preserved on Normal to Error transition

* Alerting to err test too

* Add a blurb to docs about this behavior

pkg/services/ngalert/eval/testing.go

@@ -80,6 +80,12 @@ func WithLabels(labels data.Labels) ResultMutator {
 	}
 }
 
+func WithValues(values map[string]NumberValueCapture) ResultMutator {
+	return func(r *Result) {
+		r.Values = values
+	}
+}
+
 type FakeLoadedMetricsReader struct {
 	fingerprints map[data.Fingerprint]struct{}
 }