Testing: Disallow duplicate test cases

[bmish]

This updates the test runner to fail when it detects duplicate test cases.

Example output:

 FAIL  test/unit/rules/no-duplicate-id-test.js
  ● no-duplicate-id › <div id={{"id-00"}}></div><div id={{"id-01"}}></div>: passes

    assert(received)

    Expected value to be equal to:
      true
    Received:
      false
    
    Message:
      detected duplicate `good` test case

Part of v4 release (#1908).

TODO:

• Remove duplicate tests found in 11 of our test files
• Add tests
• Fix false positives with non-serializable rule configs in duplicate test case check #2301