Unit and integration tests for the vacuum command

[rtyler]

Our preliminary support for vacuum would benefit from some more unit and integration tests to ensure that it's only deleting the right files whenever the vacuum is invoked.

[danmx]

Referencing @mrk-its comment from #97 so it's not lost.

@rtyler @fvaleye It looks like there are still two serious issues with vacuum implementation:

* vacuum lists all files in dataset using `StorageBackend.list_objs`. The problem is this function returns all files (including these in subdirectories) on `s3` backend and `gcs` backend (althrough I'm not sure about gcs). On `file` and `azure` backends this function  lists only first-level files (without recursing to subdirectories).

* vacuum ignores files not referenced by delta log at all (so not included on DeltaTableState.files() and DeltaTableState.all_tombstones() lists).