test/py: use actual core count for parallel builds

When building U-Boot we should not blindly use make -j8 but consider the
actual core count given by os.cpu_count().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>