tests/qapi-schema: Rewrite simple union TestIfUnion to be flat

Simple unions predate flat unions.  Having both complicates the QAPI
schema language and the QAPI generator.  We haven't been using simple
unions in new code for a long time, because they are less flexible and
somewhat awkward on the wire.

To prepare for their removal, rewrite TestIfUnion to be flat.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210917143134.412106-18-armbru@redhat.com>

tests/qapi-schema/qapi-schema-test.json

@@ -229,8 +229,10 @@
   'if': 'TEST_IF_ENUM' }
 
 { 'union': 'TestIfUnion',
+  'base': { 'type': 'TestIfEnum' },
+  'discriminator': 'type',
   'data': { 'foo': 'TestStruct',
-            'bar': { 'type': 'str', 'if': 'TEST_IF_UNION_BAR'} },
+            'bar': { 'type': 'UserDefZero', 'if': 'TEST_IF_ENUM_BAR'} },
   'if': { 'all': ['TEST_IF_UNION', 'TEST_IF_STRUCT'] } }
 
 { 'command': 'test-if-union-cmd',

tests/qapi-schema/qapi-schema-test.out

@@ -257,19 +257,15 @@ enum TestIfEnum
     member bar
         if TEST_IF_ENUM_BAR
     if TEST_IF_ENUM
-object q_obj_TestStruct-wrapper
-    member data: TestStruct optional=False
-enum TestIfUnionKind
-    member foo
-    member bar
-        if TEST_IF_UNION_BAR
+object q_obj_TestIfUnion-base
+    member type: TestIfEnum optional=False
     if {'all': ['TEST_IF_UNION', 'TEST_IF_STRUCT']}
 object TestIfUnion
-    member type: TestIfUnionKind optional=False
+    base q_obj_TestIfUnion-base
     tag type
-    case foo: q_obj_TestStruct-wrapper
-    case bar: q_obj_str-wrapper
-        if TEST_IF_UNION_BAR
+    case foo: TestStruct
+    case bar: UserDefZero
+        if TEST_IF_ENUM_BAR
     if {'all': ['TEST_IF_UNION', 'TEST_IF_STRUCT']}
 object q_obj_test-if-union-cmd-arg
     member union-cmd-arg: TestIfUnion optional=False