Add unit test for firewall stage

Refs: #405

internal/osbuild/firewall_stage_test.go

@@ -0,0 +1,16 @@
+package osbuild
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewFirewallStage(t *testing.T) {
+	expectedFirewall := &Stage{
+		Name:    "org.osbuild.firewall",
+		Options: &FirewallStageOptions{},
+	}
+	actualFirewall := NewFirewallStage(&FirewallStageOptions{})
+	assert.Equal(t, expectedFirewall, actualFirewall)
+}