Change moco mapping rule in Junit @Test method

Hi Zheng Ye (@dreamhead),

I have been using the `moco` during my test recently, it really helps me a lot, thx for the wonderful `moco` framework. :+1: But I notice a small issue, when I want to setup a runner in `@BeforeClass` method, and put the mapping mock rule into the `@Test` method, it seems that it will not map the mock rule correctly, see test case in details. Is this an intended behaviour from `moco` such that you can only setup mapping mock rule before `@Test` method? 

I have added and a `MocoRunnerBeforeClassTest.java` test file to demonstrate my issue. I have added an Eclipse plugin for `gradle` as I am an Eclipse user. :smile: 

Thank you very much.

Best Regards,
GaoXiang