Union branch names should use '-', not '_'.  Enforce this.  The only
offenders are in tests/.  Fix them.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-29-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message typo fixed]