Tracks testEnd arrivals per test fullName in this group's run. Reset in lockstep with the run counter — explicitly NOT on every WS 'connection' event, which was the bug that broke no-html-test in CI run 26042614416.

The WS testEnd handler enforces "QUnit fires testEnd exactly once per registered test per run" by checking this map before incrementing the counter: a second arrival of the same fullName is dropped with a # [qunitx] WARNING: duplicate testEnd ignored ... line on stderr+stdout so the underlying browser/runtime bug stays visible while pass counts stay correct. Per-group because two groups can legitimately share a fullName when they bundle different files registering the same module/test names — the dedup key is intra-group.