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/select/pytest_select.py

@@ -25,7 +25,7 @@ def get_uart_msgs(i: int) -> List[str]:
 @pytest.mark.generic
 def test_examples_select(dut: Dut) -> None:
 
-    dut.expect_exact('cpu_start: Starting scheduler', timeout=30)
+    dut.expect_exact('main_task: Calling app_main()', timeout=30)
 
     exp_list = []
     for i in range(1, 10):