Panels: Skip re-rendering panel/visualisation in loading state (#19518)

* Loading states and partial rendering, set loading state in mixed data source, and do not render loading states for react panels

* Updated mixed data source tests

public/test/mocks/datasource_srv.ts

@@ -40,7 +40,11 @@ export class MockDataSourceApi extends DataSourceApi {
     if (this.queryResolver) {
       return this.queryResolver;
     }
-    return Promise.resolve(this.result);
+    return new Promise(resolver => {
+      setTimeout(() => {
+        resolver(this.result);
+      });
+    });
   }
 
   testDatasource() {