Use testcontainers to run etcd

[AlexanderYastrebov]

Is your feature request related to a problem? Please describe.

Currently etcd tests launch standalone binary, see https://github.com/zalando/skipper/blob/master/etcd/etcdtest/mocketcd.go

Describe the solution you would like

Launch etcd via testcontainers similar to Redis, see #1960

Would you like to work on it?
Yes

[remychantenay]

@AlexanderYastrebov do you still plan on working on this?

[szuecs]

@remychantenay if you want to contribute, then please open a PR.