buildctl: add test for usage

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

cmd/buildctl/buildctl_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/moby/buildkit/util/testutil/integration"
+	"github.com/stretchr/testify/require"
 )
 
 func TestCLIIntegration(t *testing.T) {
@@ -13,5 +14,14 @@ func TestCLIIntegration(t *testing.T) {
 		testBuildLocalExporter,
 		testBuildContainerdExporter,
 		testPrune,
+		testUsage,
 	})
 }
+
+func testUsage(t *testing.T, sb integration.Sandbox) {
+	t.Parallel()
+
+	require.NoError(t, sb.Cmd().Run())
+
+	require.NoError(t, sb.Cmd("--help").Run())
+}