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

This reverts commit 1853411d8376570295711f9084d494d458822578.

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 <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>

cloudflare/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,golang/go,CAFxX/go,golang/go