cmd/compile/internal/syntax: don't hardwire path separator in test

Windows uses '\' not '/'.

Fixes #35175.

Change-Id: Ib3d01dcf148fc0675496d5213f5bcc9cf210a6fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/203889Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/syntax/parser_test.go

@@ -96,7 +96,7 @@ func walkDirs(t *testing.T, dir string, action func(string)) {
 			}
 		} else if fi.IsDir() && fi.Name() != "testdata" {
 			path := filepath.Join(dir, fi.Name())
-			if !strings.HasSuffix(path, "/test") {
+			if !strings.HasSuffix(path, string(filepath.Separator)+"test") {
 				dirs = append(dirs, path)
 			}
 		}