Tempo: fix test data source (#28836)

public/app/plugins/datasource/tempo/datasource.ts

@@ -79,13 +79,12 @@ export class TempoDatasource extends DataSourceApi<TempoQuery> {
     try {
       await this._request(`/api/traces/random`).toPromise();
     } catch (e) {
-      // As we are not searching for a valid trace here this will definitely fail but we should return 502 if it's
-      // unreachable. 500 should otherwise be from tempo it self but probably makes sense to report them here.
-      if (e?.status >= 500 && e?.status < 600) {
+      // If all went well this request will get back with 400 - Bad request
+      if (e?.status !== 400) {
         throw e;
       }
     }
-    return true;
+    return { status: 'success', message: 'Data source is working' };
   }
 
   getTimeRange(): { start: number; end: number } {