tests: mqtt: disable debug log level

Avoid to use debug log level for all modules and keep it only for MQTT
and NET related modules.

mr_canhubk3 board will fail to initialize the on-board watchdog within
the expected window causing a board reset, due to the amount of log
messages being printed. Other boards may also be affected.

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>