Test: Introduce order independent testId to replace testNumber

This is a POC. Bikeshed please.
Understood. I think it is important to implement even a simple hashing algorithm correctly, not because the consequences of failure are dire here but from an academic appreciation for these primitives (e.g., minimizing collisions by keeping all relevant output) and making their nature and purpose clear to end users (e.g., through fixed-length hex representation).

The good news, though, is that doing so is achievable. https://github.com/jquery/qunit/pull/661/files#r17518834 (updated) should address both concerns, and I didn't find any collisions when I tried it.