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>

src/cmd/go/testdata/script/list_test_e.txt

 # issue 25980: crash in go list -e -test
 go list -e -test -f '{{.Error}}' p
-stdout '^p/d_test.go:2:8: cannot find package "d" in any of:'
+stdout '^p[/\\]d_test.go:2:8: cannot find package "d" in any of:'
 
 -- p/d.go --
 package d