cmd/go: convert TestAccidentalGitCheckout to a script test

cmd/go: convert TestAccidentalGitCheckout to a script test

As far as I can determine, this is the only existing non-script test
that relies on vcs-test.golang.org. I am attempting to eliminate the
vcs-test server, and to keep configuration straightforward I would
like to only set up its replacement for script tests.

For #27494.

Change-Id: Ib978228b9c3f403af5f1c445e08942cbe7915b62
Reviewed-on: https://go-review.googlesource.com/c/go/+/418103
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>