tests rtio_i2c: Initialize stack buffers to be printed

These buffers will eventually be logged/printed by i2c_dump_msgs_rw().
Let's initialize them to anything, so we do not print garbage stack
content.
Printing garbage is not much of a problem in itself, but it trips
valgrind, and it is easy to fix.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>