Prometheus: add tests covering the prometheus-response to dataframes conversion (#43892)

* prometheus: add tests covering prometheus_response to grafana_dataframes

* fixed variable name

* added comment

* removed not-helping function call

pkg/tsdb/prometheus/testdata/range_missing.query.json

@@ -0,0 +1,8 @@
+{
+  "RefId": "A",
+  "RangeQuery": true,
+  "Start": 1641889530,
+  "End": 1641889538,
+  "Step": 1,
+  "Expr": "test1"
+}