pytest: Expect "Calling app_main()" to indicate start of application

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.

examples/system/eventfd/pytest_eventfd.py

@@ -17,7 +17,7 @@ from pytest_embedded import Dut
 @pytest.mark.generic
 def test_eventfd(dut: Dut) -> None:
 
-    dut.expect_exact('cpu_start: Starting scheduler')
+    dut.expect_exact('main_task: Calling app_main()')
 
     exp_list_5seconds = [
         'eventfd_example: Select timeouted for 1 times',