tests: use the modern config setting to enable evolution in topic tests When adding `hg pick --config extensions.evolve=` to test-topic-dest.t to investigate issue6406, it failed with `abort: unknown command 'pick'`. This is a manifestation of issue6039. I made several attempts at determining the root cause, and while I'm still worried about *why* a config setting can cause the extension to fail to fully initialize, there are some topic bugs to cleanup and I don't feel like spending more time on this.