Lets us run multifile tests and tests with arguments. R=golang-dev, r CC=golang-dev https://golang.org/cl/5753068
R=rsc CC=golang-dev https://golang.org/cl/5338043
but with less precision than hardware counterparts. fixed a number of tests to output BUG when they failed. changed the runner to distinghuish between output and output containing ^BUG R=rsc CC=dho, golang-dev https://golang.org/cl/1778041
R=iant, ken2, r, r2, ken3 CC=golang-dev https://golang.org/cl/1686044