Drop tests that are specifically about simple unions:

* SugaredUnion in doc-good: flat unions are covered by @Object.

* union-branch-case and union-clash-branches: branch naming for flat
  unions is enforced for the tag enum instead, which is covered by
  enum-member-case and enum-clash-member.

* union-empty: empty flat unions are covered by flat-union-empty.

Rewrite the remainder to use flat unions: args-union, bad-base,
flat-union-base-union, union-branch-invalid-dict, union-unknown.

Except drop union-optional-branch. because converting this one is not
worth the trouble; we don't explicitly check names beginning with '*'
in other places, either.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-21-armbru@redhat.com>