Simplify internals

[kegsay]

The idea of running docker containers in Go tests isn't new. It was reasonably new back when Complement started, but now there are libraries we could use instead, notably https://github.com/testcontainers/testcontainers-go

We should consider using this internally. This would be transparent to tests as they have a separate API shape, and greatly simplify the burden of maintaining Complement internals, as the cost of an extra dependency. This dependency though is backed by a number of companies and does test containers for other languages as well, not just Go.