Tests types run in non-deterministic order

[gsnedders]

I feel like I reported this sometime before, but we're once again in a state where ./wpt run --metadata infrastructure/metadata --manifest MANIFEST.json PRODUCT infrastructure runs tests in a non-deterministic order, because we run the test types in a non-deterministic order.

We used to always guarantee a consistent ordering to avoid any risk of flakiness due to ordering, and this potentially is now worse with #40556 having landed.