test: wait for stream close before writing to file

Wait for async operations on a file to finish before writing to it
again.

This fixes flakiness in parallel/test-readline-async-iterators-destroy.

PR-URL: #30836
Fixes: #30660
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

test/parallel/test-readline-async-iterators-destroy.js

@@ -2,6 +2,7 @@
 
 const common = require('../common');
 const fs = require('fs');
+const { once } = require('events');
 const { join } = require('path');
 const readline = require('readline');
 const assert = require('assert');
@@ -45,6 +46,8 @@ async function testSimpleDestroy() {
 
     rli.close();
     readable.destroy();
+
+    await once(readable, 'close');
   }
 }
 
@@ -78,6 +81,8 @@ async function testMutualDestroy() {
 
     rli.close();
     readable.destroy();
+
+    await once(readable, 'close');
   }
 }