Add a test for serialisation json msgs

[lindseydew]

What does this change?

These tests are a duplicate of these unit tests in the com.gu.mobile.notifications.client.models package. These have been added to this repo because we had an incident where the json serialization was broken because there was an issue with build defintion resolving the correct version of play json This test here is to add an extra check to C.I to catch this potential issue before a release

How to test

How can we measure success?

Have we considered potential risks?

Images

Accessibility

• [ ] Tested with screen reader
• [ ] Navigable with keyboard
• [ ] Colour contrast passed
• [ ] The change doesn't use only colour to convey meaning