[go] go vet report unknown identifier for example

Happened in https://travis-ci.org/dyweb/gommon/jobs/484967755 in #106 , not sure why I didn't meet it before ... 

A workaround seems to be put the example tests under the same package (no `_test` suffix)

Ref

- https://github.com/gojp/goreportcard/issues/113
- https://github.com/alecthomas/gometalinter/issues/218