graphite: use a query when testing data source

Using a query (POST /render) request instead of GET /metrics/find to better
mimic an actual request for rendering a panel in a dashboard. The POST /render
request will add at least a custom http header which can be problematic when
using direct access mode since it will trigger a CORS prerequest check. By
doing this kind of query when testing the data source possible CORS issues
can be detected there instead of later when trying to use the data source
in a dashboard.

public/app/plugins/datasource/graphite/datasource.ts

@@ -453,7 +453,13 @@ export function GraphiteDatasource(instanceSettings, $q, backendSrv, templateSrv
   };
 
   this.testDatasource = function() {
-    return this.metricFindQuery('*').then(function() {
+    let query = {
+      panelId: 3,
+      rangeRaw: { from: 'now-1h', to: 'now' },
+      targets: [{ target: 'constantLine(100)' }],
+      maxDataPoints: 300,
+    };
+    return this.query(query).then(function() {
       return { status: 'success', message: 'Data source is working' };
     });
   };