Report description for test failure when possible

extensions/ql-vscode/src/cli.ts

@@ -104,6 +104,7 @@ export interface TestCompleted {
   evaluationMs: number;
   expected: string;
   diff: string[] | undefined;
+  failureDescription?: string;
 }
 
 /**

extensions/ql-vscode/src/test-adapter.ts

@@ -223,8 +223,8 @@ export class QLTestAdapter extends DisposableObject implements TestAdapter {
           ? 'errored'
           : 'failed';
       let message: string | undefined;
-      if (event.diff?.length) {
-        message = ['', `${state}: ${event.test}`, ...event.diff, ''].join('\n');
+      if (event.failureDescription || event.diff?.length) {
+        message = ['', `${state}: ${event.test}`, event.failureDescription || event.diff?.join('\n'), ''].join('\n');
         testLogger.log(message);
       }
       this._testStates.fire({