Fix 'shell-tests-split-string' on MS-Windows

* test/lisp/shell-tests.el (shell-tests-split-string): Skip test
that always fails on MS-Windows/MS-DOS.

test/lisp/shell-tests.el

@@ -56,7 +56,8 @@
                  '("ls" "/tmp/foo bar")))
   (should (equal (split-string-shell-command "ls /tmp/'foo\\ bar'")
                  '("ls" "/tmp/foo\\ bar")))
-  (should (equal (split-string-shell-command "ls /tmp/foo\\ bar")
-                 '("ls" "/tmp/foo bar"))))
+  (unless (memq system-type '(windows-nt ms-dos))
+    (should (equal (split-string-shell-command "ls /tmp/foo\\ bar")
+                   '("ls" "/tmp/foo bar")))))
 
 ;;; shell-tests.el ends here