I was hitting intermittent failures in the test suite when running with

pytest-xdist. It turns out that three of the certificate manager tests

were polluting the in-memory siteconfig object without cleaning up

after:

test_storage_backend_with_custom
test_storage_backend_after_setting_change
test_storage_backend_with_missing_backend

This change adds to the tearDown method to clean up the offending

state.

Ran unit tests with xdist a few times to verify that we no longer had

state leakage.