test: log errors in test-fs-readfile-tostring-fail

The test writes out a large file via `fs.createWriteStream()` but was
not listening for the `error` event, which the `fs` docs describe as the
reliable way to detect write errors.

PR-URL: #27058
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/pummel/test-fs-readfile-tostring-fail.js

@@ -21,6 +21,8 @@ const stream = fs.createWriteStream(file, {
   flags: 'a'
 });
 
+stream.on('error', (err) => { throw err; });
+
 const size = kStringMaxLength / 200;
 const a = Buffer.alloc(size, 'a');