test: limit runoutput tests on arm platforms

runoutput styles tests generally consume a lot of memory. On arm platforms rotate?.go consume around 200mb each to compile, and as tests are sorted alphabetically, they all tend to run at once.

This change limits the number of runoutput jobs to 2 on arm platforms.

R=minux.ma, remyoudompheng, bradfitz, lucio.dere
CC=golang-dev
https://codereview.appspot.com/7099047