test: ensure never settling promises are detected

PR-URL: https://github.com/nodejs/node/pull/50318
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Moshe Atlow <moshe@atlow.co.il>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-fs-read-empty-buffer.js

@@ -37,5 +37,5 @@ assert.throws(
       message: 'The argument \'buffer\' is empty and cannot be written. ' +
                'Received Uint8Array(0) []'
     }
-  );
+  ).then(common.mustCall());
 })().then(common.mustCall());