func execute(testName string) ([]byte, error) {
	reTestName := fmt.Sprintf("^(%s)$", testName)
	args := append(defaultArgs, reTestName)
	return exec.Command(goBinary, args...).Output()
}
Execute a "go test" on an example testcase and return the output.
We have to do this because the testing.T package doesn't export a lot of
methods required to use it directly.
package testdeck
import (
	"fmt"
	"os/exec"
	"strings"
	"testing"
)
const examplePackage = "github.com/mercari/testdeck/demo"
const goBinary = "go"
var defaultArgs = []string{"test", "-timeout", "30s", "-v", examplePackage, "-run"}