d/tests: run rsync tests over ssh

Add remote-tests autopkgtest to d/tests/control which setups a ssh server on localhost, creates a rsync user, and then calls the local-tests script as the rsync user with the REMOTE env var set.

The local-tests autopkgtest script is adapted in such a way that if the REMOTE env var is set, it prepends a constructed REMOTE_PATH (by default empty such that the local tests run locally) to the target name. Using this feature smartly will ensure that all tests defined in local-tests are also tested over the ssh connection without having to duplicate code for test cases.