[acceptance-tests] Move git "submodules" to acceptance-tests/external instead of...

[acceptance-tests] Move git "submodules" to acceptance-tests/external instead of next to Mono repo

A few people told me it's surprising that acceptance-tests checks out repos into <mono repo>/../
and it can cause unwanted changes when a repo with the same name already exists there.

While this structure is used in several internal repos I agree for Mono it's better to keep
everything inside the repo itself.