Tests for `uniqueItems` only test arrays with two items

[DrGFreeman]

The tests for non-unique items with uniqueItems enabled only test arrays with two items, e.g.:

JSON-Schema-Test-Suite/tests/draft2020-12/uniqueItems.json

Lines 62 to 66 in 329efe5

	{
	"description": "non-unique array of arrays is invalid",
	"data": [["foo"], ["foo"]],
	"valid": false
	},

In some validation library implementations, having unique first two first items can result in the validation to pass (e.g.: python-jsonschema/jsonschema#866). In this case, addition of a third item in the array would allow testing that the validation is performed on all elements of the array:

{
    "description": "non-unique array of arrays is invalid",
    "data": [["foo"], ["bar"], ["foo"]],
    "valid": false
},

Also, there is no test case for non-unique string items (only numeric and array items).

[DrGFreeman]

I will submit a PR to add / modify tests for this issue shortly.