test_apps: move PanicTestDut from conftest into a separate module

pytest_panic used to do 'from conftest import PanicTestDut'. This
stopped working when another conftest.py file was added in
tools/test_build_system/conftest.py.
In this case we can't rename conftest.py since both are necessary
for pytest to install the hooks in the respective test cases.
Fix by moving PanicTestDut into a separate module, then importing
it from there.

tools/test_apps/system/panic/pytest_panic.py

@@ -6,8 +6,7 @@ from pprint import pformat
 from typing import List, Optional
 
 import pytest
-
-from conftest import PanicTestDut
+from test_panic_util import PanicTestDut
 
 CONFIGS = [
     pytest.param('coredump_flash_bin_crc', marks=[pytest.mark.esp32, pytest.mark.esp32s2]),