cmd/go: fix TestScript/list_test_e on Windows

I forgot to run trybots on CL 123758, and the test failed on Windows because I
hard-coded a slash-delimited path.

Use the tent-in-a-box operator ([/\\]) to make the path platform-agnostic.

Change-Id: I9113ab60d21152c11e2ebdf822b58a44b1b38574
Reviewed-on: https://go-review.googlesource.com/125115


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>