you can create a wrapper for FormsAuthentication and close it
formsAuthentication = mockery.Stub<IFormsAuthentication>();
and do something like this.
With.Mocks(mockery)
.Expecting(() => Expect.Call(() => formsAuthentication.SignOut()))
.Verify(() => controller.LogOff());
FormsAuthentication. - , .
, SignOut , . , , . Submit, .