This removes outdated TODOs and adds a test for invalid input in
`fs.copyFile` and solves a TODO by doing so.

PR-URL: #20319
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>