[test] add speed tests for gpg/wrapper init/enc/dec

this is a port of commit 2d9bec78f in the legacy keymanager repo.

Additionally, I'm here separating benchmarking tests from the main tox
run.

In my machine several of these benchmarking tests seem to be broken, due
to issue https://github.com/isislovecruft/python-gnupg/issues/157