tests/qapi-schema: Purge simple unions from tests

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>

tests/qapi-schema/doc-good.out

@@ -32,21 +32,6 @@ object Object
     case two: Variant2
         if {'any': ['IFONE', 'IFTWO']}
     feature union-feat1
-object q_obj_Variant1-wrapper
-    member data: Variant1 optional=False
-object q_obj_Variant2-wrapper
-    member data: Variant2 optional=False
-enum SugaredUnionKind
-    member one
-    member two
-        if IFTWO
-object SugaredUnion
-    member type: SugaredUnionKind optional=False
-    tag type
-    case one: q_obj_Variant1-wrapper
-    case two: q_obj_Variant2-wrapper
-        if IFTWO
-    feature union-feat2
 alternate Alternate
     tag type
     case i: int
@@ -149,13 +134,6 @@ doc symbol=Object
 
     feature=union-feat1
 a feature
-doc symbol=SugaredUnion
-    body=
-
-    arg=type
-
-    feature=union-feat2
-a feature
 doc symbol=Alternate
     body=