Commit 65d163e changed the textual message of an exception thrown by
ScheduledAnnotationBeanPostProcessor.afterSingletonsInstantiated(), and
this in turn caused the withAmbiguousTaskSchedulers_andSingleTask()
method in EnableSchedulingTests to start failing (albeit only during
'Performance' builds).
This commit updates the assertion to match the current implementation of
ScheduledAnnotationBeanPostProcessor.