os: use an actual RemoveAll failure in TestRemoveAllWithMoreErrorThanReqSize

Previously we injected an error, and the injection points were
(empirically) not realistic on some platforms.

Instead, we now make the directory read-only, which (on most
platforms) suffices to prevent the removal of its files.

Fixes #35117
Updates #29921

Change-Id: Ica4e2818566f8c14df3eed7c3b8de5c0abeb6963
Reviewed-on: https://go-review.googlesource.com/c/go/+/203502


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>