Fixed various Dropbox issues + unit tests

tldr; ran unit tests, found broken stuff, fixed stuff. unit tests all go.

- fixed touch function to return true on success
- fixed local metadata cache to remove deleted files/directories
- fixed getMetaData() to ignore files reported as deleted by
  Dropbox
- fixed "file not found" case to not log as exception
- fixed "overwrite on rename" case
- fixed unit tests to exclude unsupported cases
- added unit test for touch return value
- unit tests for Dropbox should all run correctly now

Please review @karlitschek @DeepDiver1975 @schiesbn @icewind1991