test/py: Rework test.py to be a different kind of wrapper

test/py: Rework test.py to be a different kind of wrapper

Now that we have moved to being based on pytest for python3 we need to
make our test.py wrapper more robust in terms of only calling python3
rather than possibly finding and using python2.  To do this, change from
execvp()'ing pytest to invoking the package itself via python.  In the
event that pytest is unavailable we still get a user-friendly error:

pkg_resources.DistributionNotFound: The 'pytest' distribution was not found and is required by the application

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>