test-trace2: mark unused argv/argc parameters

test-trace2: mark unused argv/argc parameters

The trace2 test helper uses function pointers to dispatch to individual
tests. Not all tests bother looking at their argv/argc parameters. We
could tighten this up (e.g., complaining when seeing unexpected
parameters), but for internal test code it's not worth worrying about.

This is similar in spirit to 126e3b3d2a (t/helper: mark unused argv/argc
arguments, 2023-03-28).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>