Backup: probe the target with a real write instead of access(W_OK)

On the server the backup folder is a Synology NFS share with "map all
users to admin": access(W_OK) returns EACCES for uid 1000 although
writes succeed, so the first scheduled run failed with "not usable".
Create and remove a probe file instead; test that it leaves nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/backup.test.js

@@ -168,6 +168,16 @@ test('backups are off without BACKUP, and a missing folder is reported, never cr
   } finally { f.cleanup(); }
 });
 
+test('the writability probe leaves nothing behind in the backup folder', async () => {
+  const f = fixture();
+  try {
+    const backup = service(f);
+    await backup.refreshTarget();
+    assert.equal(backup.publicState().targetError, null);
+    assert.deepEqual(fs.readdirSync(f.target).filter(name => name.startsWith('.circus-backup-probe')), []);
+  } finally { f.cleanup(); }
+});
+
 test('only one run at a time', async () => {
   const f = fixture();
   try {