Generic external storage integration tests

[PVince81]

Write integration tests for most file operations to be done on an external storage.

Such tests could be reused for any external storage implementation.

This should be a replacement for the current unit tests in some storages that connect directly to an external system. Normally unit tests aren't supposed to connect to external systems.

See https://github.com/owncloud/core/blob/v10.0.0/tests/lib/Files/Storage/Storage.php to get an idea what is currently tested through unit tests.

@owncloud/qa @DeepDiver1975

[SergioBertolinSG]

This will be started in WND repo. After it the other external storages can be similar. But not the same. Also each external storage will be in their own repo with their own tests.