summary.py: Change skip to N/A for tests not run

When comparing two test results the current behavior sets a status of
skip for tests that were not run in one set of results. This is
confusing and silly, so instead give them a status of N/A which gives a
clear understanding of what skipped, and what simply didn't run either
by exclusion or because it didn't exist yet.

Signed-off-by: Dylan Baker <baker.dylan.c@gmail.com>