image history enum test (#47)

adds tests for image history enum
tests both valid and invalid cases

---

### **Checklist**
- [x] Tests added/updated
- [ ] Documentation updated (if needed)
- [x] Code conforms to style guidelines

toolkit/tools/imagecustomizerapi/imagehistory_test.go

@@ -0,0 +1,20 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+package imagecustomizerapi
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestImageHistoryIsValidValid(t *testing.T) {
+	err := ImageHistoryNone.IsValid()
+	assert.NoError(t, err)
+}
+
+func TestImageHistoryIsValidInvalid(t *testing.T) {
+	err := ImageHistory("aaa").IsValid()
+	assert.ErrorContains(t, err, "invalid imageHistory value (aaa)")
+}