iotests: Use _rm_test_img for deleting test images

Just rm will not delete external data files.  Use _rm_test_img every
time we delete a test image.

(In the process, clean up the indentation of every _cleanup() this patch
touches.)

((Also, use quotes consistently.  I am happy to see unquoted instances
like "rm -rf $TEST_DIR/..." go.))

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-id: 20191107163708.833192-16-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>