ztest: fix ztest thread priority in cooperative mode
ztest: fix ztest thread priority in cooperative mode

By default ztest thread is running at the priority `-1`. This value is
invalid when the testcase is running in cooperative mode only. Set
default ztest thread priority to `-2` if this is the case. The fix is
modeled on the approach used to define the default
`MAIN_THREAD_PRIORITY`.

Signed-off-by: Piotr Mienkowski <be7d9ab625d3cb5e3a4a3202f3529b954dba084c@gmail.com>

zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr