so that GO_TEST_TIMEOUT_SCALE can be applied too.
It's for the mips64 builder, which is so slow that the
go1 benchmark can't finish startup within 10 minutes.

Change-Id: I1b824eb0649460101b294fb442da784e872403e7
Reviewed-on: https://go-review.googlesource.com/16901Reviewed-by: Ian Lance Taylor <iant@golang.org>