fleettest: add a per-target max_retry budget for flaky tests

fleettest: add a per-target max_retry budget for flaky tests

A slow or heavily-loaded fleet box can occasionally flake a concurrency-
sensitive test (e.g. a daemon/lsh test under -j8 on a nested-VM Solaris box).
Rather than dropping the whole target to a lower -j, add a per-target
"max_retry" property: after a run, each failed test is re-run on its own up to
max_retry more times, and any that then pass are dropped from the failure list.
Recovered tests are listed in a new "RECOVERED" report section, so a flake is
surfaced, never silently hidden.

Applies to every pass for the target (pipe, tcp, protoNN, nonroot).  Default 0
keeps the current no-retry behaviour.