Time the other filters at two lengths as well

The avatar checks held a 40000 byte address to half a second, twenty
times tighter than the readme budget that already failed on a slow
host, and in stub mode nearly all of that time went on the fake digest.
Link-commits and link-trailers had no check on a long input at all. The
readme checks' method moves into the harness so all four share it.

A single timing of each length let a busy host that took the CPU away
during the longer one make a linear pass look quadratic. The lengths
now take turns and the fastest timing of each counts, since a busy host
can slow one timing but rarely all five. A ratio under six ends the
turns at once, so only one near the bound of ten waits for all five.

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