cmd/dist: increase default cmd/go test timeout
cmd/dist: increase default cmd/go test timeout

cmd/go has grown slow, even in short mode, and it's now regularly
failing on a number of builders where it's taking over the previous 3
minute timeout. for now, give it more time.

Change-Id: If565baf71c2770880b2e2139b47e03433951331f
Reviewed-on: https://go-review.googlesource.com/84235
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,akutz/go,Cofyc/go,golang/go,cloudflare/go,nawawi/go,Cofyc/go,akutz/go,josharian/go,nawawi/go,cloudflare/go,cloudflare/go,akutz/go,josharian/go,Cofyc/go,codestation/go,Cofyc/go,codestation/go,codestation/go,CAFxX/go,josharian/go,nawawi/go,cloudflare/go,josharian/go,CAFxX/go,Cofyc/go,cloudflare/go,CAFxX/go,cloudflare/go,josharian/go,golang/go,akutz/go,golang/go,codestation/go,codestation/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,codestation/go,josharian/go,codestation/go,Cofyc/go,akutz/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,josharian/go,Cofyc/go,akutz/go,akutz/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,christopher-henderson/Go,josharian/go,akutz/go,Cofyc/go,CAFxX/go,cloudflare/go,codestation/go,golang/go