test: improve assertion in test-performance

PR-URL: #20809
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>

Author: addaleax

test/sequential/test-performance.js

@@ -60,7 +60,9 @@ function checkNodeTiming(props) {
       const delta = performance.nodeTiming[prop] - props[prop].around;
       assert(Math.abs(delta) < 1000);
     } else {
-      assert.strictEqual(performance.nodeTiming[prop], props[prop]);
+      assert.strictEqual(performance.nodeTiming[prop], props[prop],
+                         `mismatch for performance property ${prop}: ` +
+                         `${performance.nodeTiming[prop]} vs ${props[prop]}`);
     }
   }
 }