twister: stats: fix platform stats

Fix wrong reporting about where testcases were executed.

We have:

INFO    - 1130 of 1130 executed test cases passed (100.00%) on 0 out of
total 860 platforms (0.00%).

which is obviously wrong.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
(cherry picked from commit cfc232fa321f903c1901d76f1b03924f68725dda)
4 files changed