testbench: note expr, file, line in _fail_unless() bailout
testbench: expand the DELAY mechanism, add it to the stats pager

Doesn't break anything. Depended on by the next changeset.
testbench: string: add finite_xfer_timeout test

Part 3 could be more thorough by testing that a longer delay also triggers the
timeout. Also, "serviced in time" should also test when the reply isn't
immediate.
testbench: string: add "short enough delay" point in xferto