[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 a754d2993db1771ca3903d0a5d0e3add1883cf9b)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/223703
    Reviewed-by: Robert Griesemer <gri@golang.org>
    Run-TryBot: Andrew Bonventre <andybons@golang.org>