Add hxlnt/perftest as testcase

[paulirish]

Randomly came across this today: https://github.com/hxlnt/perftest

I threw it online here: https://frail-squirrel.surge.sh/

It seems to be mostly a stress test for paint/gpu with a lil bit of Recalc Style. Notably, there's no layout in the lifecycle here.

Might be interesting to keep around..