selftests/lxd: add simple ping test for LXD container

The current tests check if appropriate profiles exists but not if they
actually work. The nsfarm-root has to work when container at least
starts. The problem is with nsfarm-internet. It is required to test if
it is possible to access the Internet or not with that profile. That is
exactly what this test does.

selftests/lxd/test_internet.py

+"""Check if nsfarm-internet really has the Internet access.
+"""
+import pytest
+from nsfarm.lxd import Container
+from nsfarm.cli import Shell
+from .test_image import BASE_IMG
+
+
+@pytest.fixture(scope="module")
+def container(connection):
+    """Base container to be used for testing.
+    """
+    with Container(connection, BASE_IMG) as cont:
+        shell = Shell(cont.pexpect())
+        shell.run("wait4network")
+        yield shell
+
+
+@pytest.mark.parametrize("server", [
+    "217.31.205.50",  # nic.cz
+    "nic.cz",
+    "turris.cz",
+    "repo.turris.cz",
+    "cloudflare.com",
+])
+def test_ping(container, server):
+    """Simple ping to various IP addresses.
+    """
+    container.run(f"ping -c 1 '{server}'")