test: update comments in run.go (cleanup)

test: update comments in run.go (cleanup)

The -G compiler option doesn't exist anymore. Update some variable
names and comments to reflect the new reality.

Change-Id: I227e9c59a01615c3a40c3869102e8045cb012980
Reviewed-on: https://go-review.googlesource.com/c/go/+/397254
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>