gio/tests: Add file-thumbnail tests

Check if thumbnails are created in the path we expect and that we can
retrieve their information, but also that we try to get the biggest size
available when multiple are available.

gio/tests/meson.build

@@ -61,6 +61,7 @@ gio_tests = {
   'data-input-stream' : {},
   'data-output-stream' : {},
   'error': {},
+  'file-thumbnail' : {},
   'fileattributematcher' : {},
   'filter-streams' : {},
   'giomodule' : {