Please make it possible to run tests without Docker

[mgorny]

We'd like to package aiomcache for Gentoo. As part of the packaging process, we need to be able to run the test suite. While we can reasonably easily spawn a local memcached instance for testing, we can't use Docker for testing (both because it can't work without direct Internet access and because it poses security issues).

Unfortunately, the test suite seems to be really tightly tied to using a Docker container. Could you please make it possible to use an externally running memcached server instead?

[Dreamsorcerer]

We can definetely take a PR, but I'm not hugely familiar with the docker parts of the test suite. I've personally not even run the docker tests locally, only in CI (the /ut/ tests can be run without docker or other services though).