tests: info: add test for post-gst_init() category registration perf

tests: info: add test for post-gst_init() category registration perf

When registering categories after gst_init() we would re-check *all*
categories against the existing GST_DEBUG patterns again, whereas
it's enough to just check the new category. Moreover, we would parse
the GST_DEBUG pattern string again and re-add that to the existing
pattern list for every newly-registered debug category, and then
check that against all categories of course. This made registering
categories after gst_init() very very slow.