test/py: fix timeout to be absolute

Currently, Spawn.expect() imposes its timeout solely upon receipt of new
data, not on its overall operation. In theory, this could cause the
timeout not to fire if U-Boot continually generated output that did not
match the expected patterns.

Fix the code to additionally impose a timeout on overall operation, which
is the intended mode of operation.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>