golang example won't run

I'm trying to add an example to a package, and run the example via go test, however the example is never run.

For example, see this gist: http://ift.tt/1EKIy3K

The gist has example_test.go:

package cow_test

import (
    cow "http://ift.tt/1EKIy3M"
)

func Example() {
    cow.Poke()
}

Yet when I run this:

# go test -v example_test.go 
testing: warning: no tests to run
PASS
ok      command-line-arguments  0.002s

However other packages from stdlib work just fine:

# cd /usr/lib/go/src/errors
# go test -v example_test.go 
=== RUN: Example
--- PASS: Example (0.00s)
PASS
ok      command-line-arguments  0.002s

What is wrong with my example?