Fix and test seedtext for ValidationPages
    
    I broke this in the f-string conversion, and it turns out there
    were no tests so I didn't catch it till I happened to notice it
    in a manual check. So let's fix it and add tests!
    
    We also correct the test for the GA milestone: we were using
    "Final" as the milestone name in the test, but that's not how it
    works in reality, the GA milestone is called "RC" now and has
    been ever since Pungi 4 arrived. Tweaking the test revealed
    some interesting behaviours in the sort helpers that I hadn't
    noticed before, so we also add an "RC" entry to the milestone
    pairs for the sort substitutions...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>