tests: print enum type members more like object type members

tests: print enum type members more like object type members

Commit 93bda4dd461 changed the internal representation of enum type
members from str to QAPISchemaMember, but we still print only a
string.  Has been good enough, as the name is the member's only
attribute of interest, but that's about to change.  To prepare, print
them more like object type members.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20181213123724.4866-4-marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>