test: minor simplification to run.go
test: minor simplification to run.go

This is a little clearer, and we're about to need the .s file list in
one more place, so this will cut down on duplication.

Change-Id: I4da8bf03a0469fb97565b0841c40d505657b574e
Reviewed-on: https://go-review.googlesource.com/c/146998
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

cloudflare/go,josharian/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,josharian/go,nawawi/go,akutz/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,akutz/go,josharian/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,josharian/go,nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,josharian/go,akutz/go,golang/go,golang/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,josharian/go,CAFxX/go,josharian/go,cloudflare/go,golang/go,josharian/go,nawawi/go,akutz/go