[release-branch.go1.13] cmd/compile/internal/syntax: don't hardwire path separator in test

Windows uses '\' not '/'.

For #35175.
Fixes #37901.

Change-Id: Ib3d01dcf148fc0675496d5213f5bcc9cf210a6fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/203889


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit a754d299)
Reviewed-on: https://go-review.googlesource.com/c/go/+/223703


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>