tests: net: trickle: Initialize test variables earlier

The cb_[1|2]_called variables were initialized just before running
the test. This was too late as in some cases the Trickle timer
was triggered before the variables were init, which then caused the
test to fail even though there was no actual timer failure.

This was seen with frdm_k64f and when CONFIG_BOOT_DELAY=1000 was set.

Fixes #8658

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>