test: accommodate AIX by watching file

Watching directories has limited support on AIX. This is documented.
Watch a file in test/async-hooks/test-fseventwrap.js to accommodate AIX.

Refs: nodejs#13577 (comment)

Author: Trott

test/async-hooks/test-fseventwrap.js

@@ -1,6 +1,6 @@
 'use strict';
-
 require('../common');
+
 const assert = require('assert');
 const initHooks = require('./init-hooks');
 const tick = require('./tick');
@@ -10,7 +10,7 @@ const fs = require('fs');
 const hooks = initHooks();
 
 hooks.enable();
-const watcher = fs.watch(__dirname, onwatcherChanged);
+const watcher = fs.watch(__filename, onwatcherChanged);
 function onwatcherChanged() { }
 
 watcher.close();