Kill gpg agent in package-test.el to avoid a race

Kill gpg agent in package-test.el to avoid a race

package-test.el should no longer fail during clean up.

* Previously a delete-directories call raced with the gpg agent's own
  cleanup process (presumably triggered by the first deletion of one of
  the agent's sockets).  As a result, it looks like the agent could
  delete one of its sockets after delete-directories had decided to
  delete the socket, but before it made the attempt, causing an
  exception.

* To fix the problem, explicitly ask gpg-connect-agent to kill the agent
  before attempting to delete the gnupg home directory.