misc/cgo/testplugin: speed up tests

misc/cgo/testplugin: speed up tests

Running test.bash goes from 30s to 10s on a linux workstation.

(The coming pkg cache work in cmd/go would presumably do the same thing,
but this makes all.bash faster today.)

Change-Id: I8c9b0400071a412fce55b386e939906bb1c1d84d
Reviewed-on: https://go-review.googlesource.com/72330
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>