[BUGFIX] Fix .zip-export on windows and add unit test

On Windows servers (xampp), the .zip-export of extensions did not
work because \ZipArchive::addFile can't add directories on Windows.

Added a unit test for the export function.

Fixes: #45140
Releases: 6.0, 6.1
Change-Id: If248bf031d3d7b621971de609b86eedc0ee05a64
Reviewed-on: https://review.typo3.org/19118
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn