test: fix flaky test-debug-prompt

Be sure to send `.exit` only once to avoid spurious EPIPE and possibly
other errors.

Fixes: #21724

PR-URL: #21826
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>

test/sequential/test-debug-prompt.js

@@ -7,9 +7,12 @@ const spawn = require('child_process').spawn;
 const proc = spawn(process.execPath, ['inspect', 'foo']);
 proc.stdout.setEncoding('utf8');
 
+let needToSendExit = true;
 let output = '';
 proc.stdout.on('data', (data) => {
   output += data;
-  if (output.includes('debug> '))
+  if (output.includes('debug> ') && needToSendExit) {
     proc.stdin.write('.exit\n');
+    needToSendExit = false;
+  }
 });