examples: run Python examples 07 and 08 as CTest smoke tests

Neither Python example was registered as a test, so `ctest -L examples`
in CI skipped them entirely -- which is how 08's missing Python node
went unnoticed.

Add a kpn_python_example() helper (gated on KPN_BUILD_PYTHON) that runs
each script with PYTHONPATH pointed at the freshly-built module, so it
does not depend on cwd or a hard-coded build/python path, and register
07 and 08. Also del the network in 07 for deterministic teardown.

examples/07_python_network/example.py

@@ -30,3 +30,8 @@ net.build()
 net.start()
 time.sleep(0.1)
 net.stop()
+
+# Drop the network deterministically: it holds the Python callable, which forms
+# a reference cycle via globals(). Deleting the global breaks it so the network
+# is reclaimed now rather than lingering to interpreter shutdown.
+del net