4.x: Separate `TestHelper.race` uses so they can run `@Isolated`

There are no failures but the race tests are there to provoke failures. Them not failing when run parallel might just indicate the failure condition wasn't triggered due to interleaving.

Separating them out and running them JUnit 6 `@Isolated` can restore faith.

Also double check for any manual racing tests that haven't been converted to `TestHelper.race` before.

PR Welcome.