This commit updates variousf pytest scripts to expect
"main_task: Calling app_main()" instead of "cpu_start: Starting scheduler" as
indicator of the start of an application.
This updates the minimal supported version of CMake to 3.16, which in turn enables us to use more CMake features and have a cleaner build system.
This is the version that provides most new features and also the one we use in our latest docker image for CI.
