Test args-name-clash covers command parameter name clash.  This
effectively covers struct member name clash as well.  The next commit
will make parameter name clash impossible.  Convert args-name-clash
from testing command to testing a struct, and rename it to
struct-member-name-clash.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-26-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message typo fixed]