conftest: Randomize default PAN_ID

Due to some issues with other border routers running we should harden the tests against that by using a random PAN_ID.

conftest.py

@@ -6,6 +6,7 @@
 See https://docs.pytest.org/en/stable/fixture.html#conftest-py-sharing-fixture-functions
 """  # noqa: E501
 
+import random
 import re
 import os
 import subprocess
@@ -29,6 +30,7 @@
 RIOTBASE = os.environ.get('RIOTBASE')
 RUNNING_CTRLS = []
 RUNNING_EXPERIMENTS = []
+DEFAULT_PAN_ID = format(random.randint(0xFFFD), "x")
 
 
 def pytest_addoption(parser):
@@ -321,6 +323,12 @@ def update_env(node, modules=None, cflags=None, port=None, termflags=None, extra
             node.env.get('DOCKER_ENVIRONMENT_CMDLINE', '')
             + f" -e 'USEMODULE={node.env['USEMODULE']}'"
         )
+    if os.environ.get('BUILD_IN_DOCKER', 0) == '1':
+        node.env['DEFAULT_PAN_ID'] = DEFAULT_PAN_ID
+        node.env['DOCKER_ENVIRONMENT_CMDLINE'] = (
+            node.env.get('DOCKER_ENVIRONMENT_CMDLINE', '')
+            + f" -e 'DEFAULT_PAN_ID={node.env['DEFAULT_PAN_ID']}'"
+        )
     if cflags is not None:
         node.env['CFLAGS'] = cflags
     if port is not None: