Fix race condition in the http2-request-parser test harness

[tokuhirom]

Problem

In t/http2-request-parser.rakutest, the test helper runs each request's checks in a start block — off the main thread — and calls ok from there. Test is not thread safe, and this produced two distinct failures:

1. Interleaved TAP output. The multiplexing tests use $count == 2, so two start blocks emit ok concurrently and the check 1..4 lines come out in a different order on every run.

2. Tests leaking past their own pass. $test-completed.keep if $current-counter + 1 == $count only waits for the last request's checks. An earlier request's ok can still be in flight, so it lands after the pass $desc that should have followed it. That shifts the test numbering, and intermittently made the final throws-like subtest fail:

   not ok 60 - Unfinished header cannot be interrupted
   

Fix

Each request records its check results into a Promise instead of calling ok. Once every request has been checked, the main thread reports the results with ok, in request order.

The failure path is deliberately left alone: the fail => True case runs inside throws-like { ... }, i.e. a subtest with a plan, so emitting partial oks on timeout would corrupt its count.

Measurements

Output of 26 parallel runs of the file, compared by hash:

	runs producing identical output
before	8 / 26 (17 distinct outputs)
after	23 / 26

The remaining 3 hit the pre-existing Promise.in(5) deadline under CPU contention from running 26 Rakudo processes at once — not the race. Run sequentially, the file is 10/10 identical before and after the change.

Tested on Rakudo v2026.07.

Note

t/http2-response-parser.rakutest has the same helper shape, and additionally reads/writes $counter inside the start block. Every test there currently uses $count == 1, so no two check blocks ever overlap and it is stable in practice (20/20 identical under the same parallel stress). Left untouched here to keep this PR focused; happy to follow up.