spelling-tests.el: actually create a temp directory

This test was creating a file, but actually needs a directory, and
fails otherwise.

* test/lisp/so-long-tests/spelling-tests.el (so-long-spelling):
Pass the :directory arg to ert-with-temp-file so we actually
create a directory.

test/lisp/so-long-tests/spelling-tests.el

@@ -52,6 +52,7 @@
     ;; when starting the inferior ispell process, so we set HOME to a valid
     ;; (but empty) temporary directory for this test.
     (ert-with-temp-file tmpdir
+      :directory t
       :suffix "so-long.ispell"
       (let* ((process-environment (cons (format "HOME=%s" tmpdir)
                                         process-environment))