test: add key processing benchmark

test: add key processing benchmark

This runs a bunch of random keys against xkb_state_update_key() and
xkb_state_key_get_one_sym(), in a fairly unintelligent way.

It might be nice to check when modifying this code path, or changing it,
to see things haven't slowed down considerably. However, given the
numbers this benchmark gives, it is pretty clear that we are not going
to be the bottleneck for anything. So this can more-or-less be ignored.

Incidentally, this also turned out to be a poor man's fuzzer, because it
turned up the fix in the previous commit. Maybe we should consider
beefing it up with an actual 'break stuff' intention and running it as
part of 'make check'.

Signed-off-by: Ran Benita <ran234@gmail.com>