This CL also re-enables the cgo tests that were accidentally disabled
in CL 32754.

Fixes #18389.

Change-Id: I2fdc4fe3ec1f92b7da3db3fa66f4e0f806fc899f
Reviewed-on: https://go-review.googlesource.com/34660Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>