[frontend] fix finished status for SRPM builds, hopefully last time
    
    There was a mistake in the test and therefore a finished status
    could be wrong without us noticing. The original test didn't really
    tested SRPM builds because even though we changed `source_status`,
    the build has build_chroots so it wasn't correctly representing
    an SRPM build.
    
    I am creating a constant defining the list of finished states,
    so it can be easily used within `finished` properties for several
    models.