Add support check mode for all info modules

As it's required by new ansible-test rules.
See https://github.com/ansible-collections/overview/issues/45#issuecomment-893543025
Change-Id: Ib6b73e810b972997b8de2b4a9eb8e07e246823d5

plugins/modules/subnets_info.py

@@ -140,6 +140,9 @@ class SubnetInfoModule(OpenStackModule):
         name=dict(required=False, default=None, aliases=['subnet']),
         filters=dict(required=False, type='dict', default=None)
     )
+    module_kwargs = dict(
+        supports_check_mode=True
+    )
 
     def run(self):
         kwargs = self.check_versioned(