fleettest: add --timing to show per-target wall-clock

fleettest: add --timing to show per-target wall-clock

Records wall-clock per phase (push, build, each test transport, nonroot)
plus a total in TargetResult, and with --timing prints a breakdown after
the report, sorted slowest-target-first. Targets run in parallel, so the
run is gated by the slowest one; the phase columns show whether that
hold-up is the push, the build, or a test pass. A target that failed
early (no total) falls back to the sum of the phases it reached.