selftests: check if devices are on system

This is just test that checks that we have all required devices on host
system. These devices are optional and additional modules are requires
to be installed and loaded to kernel.

At the moment only the /dev/ppp is tested there.

selftests/test_local_devices.py

+import pytest
+import pathlib
+
+
+@pytest.mark.parametrize("device", [
+    "/dev/ppp",
+])
+def test_local_device(device):
+    """Check that we have local device required for testing.
+    """
+    assert pathlib.Path(device).exists()