subscription: allow dates in ISO 8601 format
    
    Right now, the date returned by D-Bus method calls of
    subscription-manager have a locale-specific format, which can be hard to
    parse. A planned change is to return all the dates in ISO 8601 format:
    https://github.com/candlepin/subscription-manager/pull/2551
    
    Hence, make the subscription module able to cope with the future value:
    first try to parse the date as ISO 8601, and if it does not work try to
    parse as it is done today. Add small unit testing data to check that
    both ISO 8601 and the locale-specific formats work.
    
    (cherry picked from commit f65f7fd4da0cdff2e358e37561925319e364f521)
    
    Resolves: rhbz#1947445