Include integration test. Confirmed that without the fix, the test case
TestDeleteReadOnly fails.

This permits to revert "cmd/go: reset read-only flag during TestIssue10952"
This reverts commit 3b7841b3.

Fixes #9606

Change-Id: Ib55c151a8cf1a1da02ab18c34a9b58f615c34254
Reviewed-on: https://go-review.googlesource.com/18235Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>