test.py: calculate bad patterns on change only

A future patch will use the bad_patterns array in multiple places. Rather
than duplicating the code to calculate it, or even sharing it in a
function and simply calling it redundantly when nothing has changed, only
re-calculate the list when some change is made to it. This reduces work.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>