Silence get_mtls_endpoint_and_cert_source is deprecated warnings in unit tests that are testing deprecated features

[mukund-ananthu]

Currently, this is going to be suppressed in the python-pubsub/pytest.ini file

[parthea]

Since this is part of the autogenerated code I'll transfer the issue to gapic-generator-python.

For more context on the issue, this line had to be added to pytest.ini in googleapis/python-pubsub#1074 to silence a warnings related to get_mtls_endpoint_and_cert_source which is deprecated

    # Remove warning once https://github.com/googleapis/gapic-generator-python/issues/1939 is fixed
    ignore:get_mtls_endpoint_and_cert_source is deprecated.:DeprecationWarning

The warnings appeared in generated tests. Assuming that the purpose of the test is to exercise the deprecated code, deprecation warnings should be silenced when testing these deprecated functions.