test: refactor test-fs-readfile-unlink

* Use tmp directory instead of mutating the fixtures directory.
* Add comment explaining that the unlinkSync() at the end of the test is
  part of the test. Otherwise it may be tempting to remove it as
  unnecessary tmp directory cleanup.

PR-URL: https://github.com/nodejs/node/pull/15173


Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>