Revert "testing/fstest: test that ReadDirFile on a non-dir fails"

This reverts commit 1853411d.

Reason for revert: broke plan 9 builder. fixes #44967

Change-Id: Ib89448d37f7ab8bb05dbd89ce744431d807eb4da
Reviewed-on: https://go-review.googlesource.com/c/go/+/301190


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>