ztest: add comment about expected thread priorities
ztest: add comment about expected thread priorities

There is an implicit expectation that the priority of threads spawned by
a ztest suite is still higher than the main suite thread own priority
when a test signals the main suite thread that it has completed.

Change-Id: Id7caec3e9e553712c828a93c212b8e82bd16eabd
Signed-off-by: Benjamin Walsh <2c289a12a296c7195a7f06fd0f3fe281728960bf@gmail.com>

erwango/zephyr,zephyriot/zephyr,bboozzoo/zephyr,holtmann/zephyr,fractalclone/zephyr-riscv,punitvara/zephyr,pklazy/zephyr,aceofall/zephyr-iotos,nashif/zephyr,mbolivar/zephyr,kraj/zephyr,punitvara/zephyr,ldts/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,runchip/zephyr-cc3220,mbolivar/zephyr,erwango/zephyr,ldts/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr,bigdinotech/zephyr,erwango/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,tidyjiang8/zephyr-doc,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,bboozzoo/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,explora26/zephyr,rsalveti/zephyr,sharronliu/zephyr,runchip/zephyr-cc3200,bboozzoo/zephyr,sharronliu/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,fbsder/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,holtmann/zephyr,runchip/zephyr-cc3220,holtmann/zephyr,finikorg/zephyr,tidyjiang8/zephyr-doc,rsalveti/zephyr,kraj/zephyr,kraj/zephyr,Vudentz/zephyr,zephyriot/zephyr,bigdinotech/zephyr,pklazy/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,rsalveti/zephyr,galak/zephyr,tidyjiang8/zephyr-doc,rsalveti/zephyr,pklazy/zephyr,erwango/zephyr,nashif/zephyr,runchip/zephyr-cc3220,punitvara/zephyr,explora26/zephyr,runchip/zephyr-cc3200,nashif/zephyr,galak/zephyr,punitvara/zephyr,runchip/zephyr-cc3200,bigdinotech/zephyr,explora26/zephyr,fractalclone/zephyr-riscv,Vudentz/zephyr,fbsder/zephyr,holtmann/zephyr,fractalclone/zephyr-riscv,tidyjiang8/zephyr-doc,finikorg/zephyr,runchip/zephyr-cc3220,galak/zephyr,GiulianoFranchetto/zephyr,aceofall/zephyr-iotos,GiulianoFranchetto/zephyr,zephyriot/zephyr,fbsder/zephyr,fractalclone/zephyr-riscv,Vudentz/zephyr,pklazy/zephyr,holtmann/zephyr,mbolivar/zephyr,ldts/zephyr,zephyriot/zephyr,bboozzoo/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,sharronliu/zephyr,rsalveti/zephyr,bigdinotech/zephyr,runchip/zephyr-cc3200,kraj/zephyr,erwango/zephyr,bboozzoo/zephyr,bigdinotech/zephyr,fbsder/zephyr,fractalclone/zephyr-riscv,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,sharronliu/zephyr,runchip/zephyr-cc3220,pklazy/zephyr,finikorg/zephyr,sharronliu/zephyr,aceofall/zephyr-iotos,tidyjiang8/zephyr-doc,aceofall/zephyr-iotos,mbolivar/zephyr