testbench: minor cleanup, callsite fix wrt Mung::Ctrl->completed

It's not an accessor to a hash reference anymore, ffs
testbench: catch and report panics

This is neither nice or clean. The testbench program has grown hirsute and
teems with the ghosts of legacy interfaces. A partial rewrite atop TAP::Parser
is in the cards.
testbench: rework to use TAP::Parser

This removes our own TAP parser, which would be perpetually insufficient
anyway, and further simplifies the Mung::TestResult class to delegate its
reporting-related accessors (besides those concerning test panics and other
abnormal exits) to TAP::Parser.

On the downside, progressive output only happens between test program runs so
e.g. fork tests can make the interactive user wait over a second. (the
horror.)
add note about library format fix to README.md