ieee488: introduce tests for unified IEEE-488 decoder (GPIB and IEC)

Test the visual annotations as well as binary and Python outputs for several
parallel GPIB captures (HP1631D, HP33120A, HP53131A, Keithley 2015) and the
serial IEC capture (CBM1571).

The "gpib_hp53131a_ton" capture does provide raw bytes, but its "payload"
data output really is empty in the current decoder's implementation. A
future implementation of the decoder may start flushing accumulated data
for currently unhandled conditions, too.