test: allow flags in run action

Previously, we could not run tests with -l=4 on NaCl since the buildrun
action is not supported on NaCl. This lets us run tests with build flags
on NaCl.

Change-Id: I103370c7b823b4ff46f47df97e802da0dc2bc7c3
Reviewed-on: https://go-review.googlesource.com/38170
Run-TryBot: David Lazar <lazard@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

test/run.go

@@ -737,6 +737,7 @@ func (t *test) run() {
 		if *linkshared {
 			cmd = append(cmd, "-linkshared")
 		}
+		cmd = append(cmd, flags...)
 		cmd = append(cmd, t.goFileName())
 		out, err := runcmd(append(cmd, args...)...)
 		if err != nil {