test: add //go:build support to run.go

test: add //go:build support to run.go

gofmt is rewriting +build comments into //go:build anyway, so update
the test script to support both.

Change-Id: Ia6d950cfaa2fca9f184b8b2d3625a551bff88dde
Reviewed-on: https://go-review.googlesource.com/c/go/+/399794
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>