Issue #3616956: Stop a click made while a request is out from being lost under CI load

A sampling branch, not a fix yet.

testClickDuringTheRequestSurvives fails about one CI run in five and has never been reproduced locally: 25 consecutive instrumented runs of the test alone, 5 of its whole class, and a dozen more under deliberate contention, all green.

So this branch samples where the failure lives. run-tests.sh accepts a single method through --class Class::method, --repeat runs it twenty times, and every other job is skipped, so one pipeline is twenty samples of one test instead of one sample of everything. A green pipeline previously said almost nothing about a one-in-five failure.

The test also carries a probe recording, at each answer: whether the second click registered and what the field became, how many requests left, what each answer said, whether the field was focused, and what max came back. Focus is the one that matters most, since the catch-up that resends a click the answer did not carry is skipped while the visitor is in the field.

Ruled out already, in [#3616893]: widening every wait past twenty seconds; clicking synchronously inside ajaxSend (which made it worse, and why); and the queue dropping the gesture, disproved by the sibling test's timeline showing it refusing correctly and then posting.

The CI configuration and the probe both come out before this is merged.