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');
@@ -42,6 +43,11 @@ async function testSimpleDestroy() {
     expectedLines.splice(1);
 
     assert.deepStrictEqual(iteratedLines, expectedLines);
+
+    rli.close();
+    readable.destroy();
+
+    await once(readable, 'close');
   }
 }
 
@@ -72,6 +78,11 @@ async function testMutualDestroy() {
     }
 
     assert.deepStrictEqual(iteratedLines, expectedLines);
+
+    rli.close();
+    readable.destroy();
+
+    await once(readable, 'close');
   }
 }