Add and modify uniqueItems tests

[DrGFreeman]

Fixes #524

Improve uniqueItems tests to ensure validators verify all elements of arrays for uniqueness:

• Use more than two items in the test arrays.
• Set the first and last items as non-unique where applicable.
• Add tests for arrays of unique and non-unique strings.

[Julian]

Thanks -- this is essentially good, but we try not to modify tests unless they're incorrect -- can you instead add the 3+ element test as a separate test rather than modifying the existing one?

And appreciated!

[DrGFreeman]

Sure! I'll add tests for the three elements instead of modifying existing ones.

[DrGFreeman]

@Julian, as per your request I've modified the PR to only add tests and not modify existing ones. Let me know if you'd like me to squash the commits into a single one. Thx.

[Julian]

Thanks! Merging, looks great.