Fixed SMB rename function to overwrite target file

When uploading files through WebDAV, a part file is created and a rename
operation is performed with the expectation that the part file
overwrites an existing file, if any.

This fix makes the SMB external storage delete the target file before
renaming, as smbclient doesn't support overwrite on move/rename.

Fixes #5348
Added unit test for "overwrite file on rename/move"

Also fixed "rename" unit test that was ready the result out of the wrong
file.
Improved unit test for "overwrite on move"

Now using a different content to make sure the file was overwritten.
Merge pull request #6008 from owncloud/extstorage-smb-webdav-renamefix

Fixed SMB rename function to overwrite target file