都是Test Double的一种，不同的是：
Stub
Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test.
stub提供的是对对象的方法response，stub是状态校验。
Mock
Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
mock期望对象接收到的规范，属于行为校验。
Reference
