build: add GOTESTONLY environment test for Plan 9's run.rc
$GOTESTONLY controls which set of tests gets run. Only "std" is
supported. This should bring the time of plan9 builder down
from 90 minutes to a maybe 10-15 minutes when running on GCE.
(Plan 9 has performance problems when running on GCE, and/or with the
os/exec package)
This is a temporary workaround for one builder. The other Plan 9
builders will continue to do full builds. The plan9 buidler will be
renamed plan9-386-gcepartial or something to indicate it's not running
the 'test/*' directory, or API tests. Go on Plan 9 has bigger problems
for now. This lets us get trybots going sooner including Plan 9,
without waiting 90+ minutes.
Update #9491
Change-Id: Ic505e9169c6b304ed4029b7bdfb77bb5c8fa8daa
Reviewed-on: https://go-review.googlesource.com/2522 Reviewed-by: Rob Pike <r@golang.org>