test: apply promises API to third appendFile test

Add tests for `fs.promises.appendFile()` to the third (of five) test
case in `test-fs-access`. (First and second test cases already have
promises API versions.)

PR-URL: https://github.com/nodejs/node/pull/21131


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>