elm_progressbar test: Add test for custom format strings to show when %% is not escaped correctly.
This adds a test that sets a custom progressbar format string that includes a custom percent (%%) that should be escaped to just one percent (%). This case has been broken and fixed many times and is broken again so it makes sense to now add a check for it to try and prevent the continual breakage.
Reviewed-by: Cedric BAIL <email@example.com>
Differential Revision: https://phab.enlightenment.org/D7745