[release-branch.go1.13] 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.
    
    Also remove unused test hook, as was done in CL 204060.
    
    For #35117.
    For #29921.
    Fixes #37895.
    
    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>
    (cherry picked from commit 06bdd52f7540eca9e3ade6e78234d00703f3ee23)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/223700
    Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
    Reviewed-by: Alexander Rakoczy <alex@golang.org>