test/py: Rework test_spi.py to assert we found output

test/py: Rework test_spi.py to assert we found output

When running a newer version of pylint it will complain that page_size
may be used before being assignment. Looking deeper what is going on is
that we could run in to the case where the regex we run for any of the
flash information fails but since we don't have a result, we don't check
it either. In the case of the rest of the numerical values we then have
some assignment (multiplying by some value) and so pylint doesn't
complain. Rework things to assert that each regex has a result and so
failure will stop the test and we won't have any use before assignment.

Signed-off-by: Tom Rini <trini@konsulko.com>