Make the nativecomp test eln directory more reliably be removed

Make the nativecomp test eln directory more reliably be removed

* lisp/startup.el (startup-redirect-eln-cache, normal-top-level):
Don't create the nativecomp directory here, because this led to
brittle deletions of the directory -- there would be several
instances of the directory left over after a test run.

* lisp/emacs-lisp/ert.el (ert-run-tests-batch-and-exit): Create
the nativecomp directory.