iotests: Make 200 run on tmpfs

200 currently fails on tmpfs because it sets cache=none.  However,
without that (and aio=native), the test still works now and it fails
before Jeff's series (on fc7dbc11).  So
we can probably remove the aio=native safely, and replace cache=none by
cache=$CACHEMODE.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 20180117135015.15051-1-mreitz@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>