Issue #85: Add test for relative symlink preservation patch.

This test adds a fixture RelativeSymlinks.zip with the following
structure:

    RelativeSymbolicLink
    ├── symlinkedFile
    ├── symlinkedFolder
    └── symlinks
       ├── fileSymlink -> ../symlinkedFile
       └── folderSymlink -> ../symlinkedFolder

The test checks if both `fileSymlink` and `folderSymlink` still point to
the respective files in the parent directory after the fixture has been
unzipped.