cmd/dist: consolidate runtime CPU tests

cmd/dist: consolidate runtime CPU tests

Instead of running:

go test -short runtime -cpu=1
go test -short runtime -cpu=2
go test -short runtime -cpu=4

Run just:

go test -short runtime -cpu=1,2,4

This is a return to the Go 1.4.2 behavior.

We lose incremental display of progress and
per-cpu timing information, but we don't have
to recompile and relink the runtime test,
which is slow.

This cuts about 10s off all.bash.

Updates #10571.

Change-Id: I6e8c7149780d47439f8bcfa888e6efc84290c60a
Reviewed-on: https://go-review.googlesource.com/9350
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>