Fix mocking code (#1372)

Embarassingly, #1368 broke the mocking code as `warnings` was never
imported. I didn't notice because `kernel.execute` doesn't report if the
execution fails. I'm making a companion PR to catch this.

scripts/nb-tester/qiskit_docs_notebook_tester/__init__.py

@@ -40,6 +40,7 @@
 
 # If not submitting jobs, we also run this code before notebook execution to mock the real backend
 MOCKING_CODE = """\
+import warnings
 from qiskit_ibm_runtime import QiskitRuntimeService
 from qiskit.providers.fake_provider import GenericBackendV2