Backend Plugins: Convert test data source to use SDK contracts (#29916)

Converts the core testdata data source to use the SDK contracts and by that 
implementing a backend plugin in core Grafana in similar manner as an external one.

Co-authored-by: Will Browne <will.browne@grafana.com>
Co-authored-by: Marcus Efraimsson <marefr@users.noreply.github.com>
Co-authored-by: Ryan McKinley <ryantxu@gmail.com>

packages/grafana-runtime/src/utils/DataSourceWithBackend.ts

@@ -112,7 +112,7 @@ export class DataSourceWithBackend<
       })
       .pipe(
         map((rsp: any) => {
-          return toDataQueryResponse(rsp);
+          return toDataQueryResponse(rsp, queries as DataQuery[]);
         }),
         catchError((err) => {
           return of(toDataQueryResponse(err));