Enable warnings in tests/test_examples.py

I just noticed that our usual pytest warnings filters (-Werror with a few exceptions) do not apply to the eamples/*.py when run by subprocess.check_call(), leading to some warnings raised by examples.

I tried sketching a possible solution in the example-warnings branch, but it appears difficult to specify exact warnings, and there are some possibly unrelated issues with torch_shm_manager and with the jit raising SystemError: ... returned a result with an error reset.