Test: Introduce order independent testId to replace testNumber

This is a POC. Bikeshed please.
Wha??? I realized this morning that it doesn't cover the "too short" case, but there should _still_ be no more or fewer collisions even without it:
```js
hex = hash.toString( 16 );
if ( hex.length < 8 ) {
	hex = "0000000" + hex;
}
return hex.slice( -8 );
```

Do you have a commit ref that I can git fetch to see what's going on?