test: improve error message in async-wrap test

Improve AssertionError message in
test/parallel/test-async-wrap-pop-id-during-load.js to include the
contents of stderr when the spawned process fails.

Refs: https://github.com/nodejs/node/pull/20940/files#r190707951

Author: Trott

test/parallel/test-async-wrap-pop-id-during-load.js

@@ -17,7 +17,8 @@ const ret = spawnSync(
   process.execPath,
   ['--stack_size=50', __filename, 'async']
 );
-assert.strictEqual(ret.status, 0);
+assert.strictEqual(ret.status, 0,
+                   `EXIT CODE: ${ret.status}, STDERR:\n${ret.stderr}`);
 const stderr = ret.stderr.toString('utf8', 0, 2048);
 assert.ok(!/async.*hook/i.test(stderr));
 assert.ok(stderr.includes('UnhandledPromiseRejectionWarning: Error'), stderr);