Add basic CLI test using Ginkgo/Gomega

Test that we can call our CLI binary with the `--help` flag and make
some basic assertions about its output.

I'll commit the new dependencies to `vendor/` in a separate commit to
make the diff easier to read.

circleci_cli_suite_test.go

@@ -0,0 +1,26 @@
+package main_test
+
+import (
+	"testing"
+
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+	"github.com/onsi/gomega/gexec"
+)
+
+var pathCLI string
+
+var _ = BeforeSuite(func() {
+	var err error
+	pathCLI, err = gexec.Build("github.com/circleci/circleci-cli")
+	Ω(err).ShouldNot(HaveOccurred())
+})
+
+var _ = AfterSuite(func() {
+	gexec.CleanupBuildArtifacts()
+})
+
+func TestCircleciCli(t *testing.T) {
+	RegisterFailHandler(Fail)
+	RunSpecs(t, "CircleciCli Suite")
+}

main_test.go

@@ -0,0 +1,23 @@
+package main_test
+
+import (
+	"os/exec"
+
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+	"github.com/onsi/gomega/gbytes"
+	"github.com/onsi/gomega/gexec"
+)
+
+var _ = Describe("Main", func() {
+	Describe("when run with --help", func() {
+		It("return exit code 0 with help message", func() {
+			command := exec.Command(pathCLI, "--help")
+			session, err := gexec.Start(command, GinkgoWriter, GinkgoWriter)
+			Expect(err).ShouldNot(HaveOccurred())
+			Eventually(session).Should(gexec.Exit(0))
+			Eventually(session.Out).Should(gbytes.Say("Usage:"))
+			Eventually(session.Err.Contents()).Should(BeEmpty())
+		})
+	})
+})