Move code from tezos-test-services to lib-test and rename

As discussed over Slack with @arvidnl , let's split tezos-test-services between generic code and Tezos code:

• Generic, Tezos-agnostic code should go to lib-test, that will be created in !2688 (merged)
• Tezos-specific code should remain in the current lib

Also, let's take this opportunity to rename tezos-test-services to lib-tezos-test (services is one of the only mistakes @arvidnl has ever done in his career 😁 )

Don't forget to document this change and its consequences on the Test documentation, e.g. http://tezos.gitlab.io/developer/testing.html