test: improve asset msg in test

Add result to part of assert message.

PR-URL: nodejs/node#15918
Reviewed-By: Ruben Bridgewater <[email protected]>

Author: genewoo

test/parallel/test-zlib-from-gzip-with-trailing-garbage.js

@@ -19,7 +19,7 @@ zlib.gunzip(data, common.mustCall((err, result) => {
   assert.strictEqual(
     result.toString(),
     'abcdef',
-    'result should match original string'
+    `result '${result.toString()}' should match original string`
   );
 }));