Test suite CI should ensure tests' schema IDs are unique

Per discussion at https://github.com/json-schema-org/JSON-Schema-Test-Suite/pull/360 and https://github.com/json-schema-org/JSON-Schema-Test-Suite/pull/509#discussion_r700744243

The test suite sanity checker should collects all the $ids used in tests and verify they're unique, to make it easier on implementers whose implementations cache references or who wish to load all references at once.