Fixed various Dropbox issues + unit tests

- 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
Merge pull request #6054 from owncloud/extstorage-dropboxfixes

Fixed various Dropbox issues + unit tests