Add the following test cases to FileAssociationsTest test:

 - mime-type="foo, foo" (multiple mimes, but the same value)
 - icon is a directory
 - icon is a path that doesn't exist
 - extension is empty string (windows only)
 - multiple extensions per mime-type (windows only)
 - the same mime and the same extension in two separate files
 - the same mime and different extension in two separate files
 - valid mime and extension in one file, the same mime and no extension in another (same/different icon, same/different extension)

macos fa has additional properties that are not tested at all. Add corresponding tests.