rabbit_access_control: Allow a testsuite to declare valid auth backend modules

[Why]
rabbit_access_control checks that the auth backend module is provided by
`rabbit` or an enabled plugin. Some testsuites use a local auth backend
module for tests purpose that do not come as a plugin. We need a
mechanism to allow that.

[How]
We introduce a new `test_auth_backends` application envoironment
variable that testsuites can used to declare their valid auth backend
modules.

Author: dumbbell

deps/rabbit/src/rabbit_access_control.erl

@@ -141,7 +141,7 @@ is_auth_backend_module_enabled(Mod) when is_atom(Mod) ->
                                "it will be skipped during "
                                "authentication/authorization",
                                [Mod, PluginName]),
-                            false
+                            is_test_auth_backend_module(Mod)
                     end;
                 {error, no_provider} ->
                     ?LOG_INFO(
@@ -150,10 +150,14 @@ is_auth_backend_module_enabled(Mod) when is_atom(Mod) ->
                        "module. Until then it will be skipped during "
                        "authentication/authorization",
                        [Mod]),
-                    false
+                    is_test_auth_backend_module(Mod)
             end
     end.
 
+is_test_auth_backend_module(Mod) ->
+    Mods = application:get_env(rabbit, test_auth_backends, []),
+    lists:member(Mod, Mods).
+
 -spec check_user_pass_login
         (rabbit_types:username(), rabbit_types:password()) ->
             {'ok', rabbit_types:user()} |