GeezerVison

…rns_unhealthy_after_the_threshold

The test asserted that reports() was still empty right after the chunk
entered its retry wait, racing a 500ms real-wall-clock threshold. On a
busy CI runner the test thread can lose the scheduler for that long
before it gets to the assertion, so the health report had already
landed. Raise the threshold to 3s to give the check a realistic margin;
the test's own 10s bound on the opposite wait still leaves plenty of
room for the report to show up.

Seen twice in the merge queue for #7736 (runs 41750935279 and
41850195763), tracked in #7828.

Signed-off-by: mythical-fred-oss[bot] <319348225+mythical-fred-oss[bot]@users.noreply.github.com>

…d one

`MockServer::start()` hands out servers from a pool and recycles them
when a test drops its server. A chunk writer that a failed test leaves
behind on the shared runtime can send one more request, which then lands
on the recycled server of whichever test picked it up next, and that
test counts it as its own. In CI, `the_retry_budget_is_exact` saw two
requests instead of one right after
`a_long_retry_loop_turns_unhealthy_after_the_threshold` failed.

`MockServer::builder().start()` returns a server that is not pooled, so
a stray request only ever reaches the server of the test that made it.

Reproduced with a panic injected after the chunk entered its retry and
the released task delayed by 100 ms: 5 of 10 runs broke another retry
test with the pooled server, 0 of 10 with this change.

Refs #7828.

The timer that reports a long retry loop unhealthy starts at the chunk's
first retry, which is after the test calls `write_batch`. So an instant
taken before that call bounds the timer: while less than the threshold
has passed since then, no report can exist yet, and the assertion holds
by construction. A runner that stalls past the threshold skips the check
instead of failing it, so the test no longer races a wall clock.

Refs #7828.