cmd/go: fix and reenable TestAccidentalGitCheckout

This is an important security problem so we shouldn't disable the test.
The second half was failing on case-sensitive file systems but the
first half is still good.

Fixes #22983.

Change-Id: I437bb4c9f78eb3177aa8b619e2357b2539566ca9
Reviewed-on: https://go-review.googlesource.com/129797


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>