T12545 is sensitive to `-dunique-increment` changes

While debugging alleged regressions in !1866 (closed), !4163 (closed) and !4902 (closed), I ultimately came to the conclusion that a +13% increase in calls to IntMap.$winsert correlates with a change in -dunique-increment numbers.

Which makes the acceptance window of +-1% inappropriate for performance testing, as the changes in allocation were +-3% with the same compiler binary.