Enable test/py for sandbox in Travis CI

This provides runtime test coverage in Travis, in addition to the existing
build coverage.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Roger Meier <r.meier@siemens.com>
Reviewed-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>