tests/qapi-schema: Improve union discriminator coverage

A union's 'discriminator' must name one of the common members.
QAPISchemaVariants.check() looks it up by its c_name(), then checks
the name matches exactly (because c_name() is not injective).

Tests union-base-empty and union-invalid-discriminator both cover the
case where lookup fails.  Repurpose the latter to cover the case where
it succeeds and the name check fails.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230316071325.492471-10-armbru@redhat.com
Reviewed-by: Eric Blake <eblake@redhat.com>
[Commit message typo fixed]

tests/qapi-schema/union-invalid-discriminator.err

@@ -1,2 +1,2 @@
 union-invalid-discriminator.json: In union 'TestUnion':
-union-invalid-discriminator.json:10: discriminator 'enum_wrong' is not a member of 'base'
+union-invalid-discriminator.json:10: discriminator 'type_tag' is not a member of 'base'

tests/qapi-schema/union-invalid-discriminator.json

@@ -8,7 +8,7 @@
   'data': { 'integer': 'int' } }
 
 { 'union': 'TestUnion',
-  'base': { 'enum1': 'TestEnum' },
-  'discriminator': 'enum_wrong',
+  'base': { 'type-tag': 'TestEnum' },
+  'discriminator': 'type_tag',
   'data': { 'value1': 'TestTypeA',
             'value2': 'TestTypeB' } }