Added tests for the interaction of default and required.

[mwadams]

Closes #834

[gregsdennis]

Need to add to v1 as well.

[jdesrosiers]

Can you add this to the v1 tests as well. Thanks. (Nevermind. Should have read Greg's review before commenting 😅)

[notEthan]

Would it be worth adding a variant with default + required keywords being siblings? e.g.

{
    "type": "object",
    "default": {"alpha": 5},
    "required": ["alpha"]
}