misc/cgo/test: fix tests in module mode

This change preserves the ability to test misc/cgo/test in GOPATH
mode, at the cost of indirection through a 'go test' subprocess.

Updates #30228

Change-Id: I08de855e62278d30fa622b2f7478e43dd2ab0e96
Reviewed-on: https://go-review.googlesource.com/c/163418


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>