misc/cgo/test: run tests

misc/cgo/test: run tests

The new gotest ignores Test functions outside *_test.go files
(the old shell script allowed them), so replace one clumsy hack
with another.

The root problem is that the package makefiles only know
how to run cgo for source files in the package proper, not
for test files.  Making it work for test files is probably more
trouble than it's worth.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4452060