[C] Add test schemas for the recent changes

The recent commit 47665aa ("Fix a few issues with the C generator
(part 1 version 2) (OpenAPITools#14434)") didn't include any test schemas. Add them
now, as requested:

  OpenAPITools#14434 (comment)

Author: eafer

modules/openapi-generator/src/test/resources/2_0/c/petstore.yaml

@@ -75,6 +75,24 @@ paths:
         - petstore_auth:
             - 'write:pets'
             - 'read:pets'
+  /pet/specialty:
+    get:
+      tags:
+        - pet
+      summary: Specialty of the shop
+      description: Returns the kind of pet the store specializes in
+      operationId: specialtyPet
+      produces:
+        - application/xml
+        - application/json
+      parameters: []
+      responses:
+        '200':
+          description: successful operation
+          schema:
+            $ref: '#/definitions/Preference'
+      security:
+        - api_key: []
   /pet/findByStatus:
     get:
       tags:
@@ -626,6 +644,17 @@ definitions:
         type: string
     xml:
       name: Category
+  Preference:
+    title: Pet preference
+    description: A user's preference in pets
+    type: string
+    enum:
+      - cats
+      - dogs
+      - birds
+      - fish
+      - snakes
+      - other
   User:
     title: a User
     description: A User who is purchasing from the pet store
@@ -650,6 +679,12 @@ definitions:
         type: integer
         format: int32
         description: User Status
+      extra:
+        type: object
+        nullable: true
+        additionalProperties: true
+      preference:
+        $ref: '#/definitions/Preference'
     xml:
       name: User
   Tag: