test cases failed because of breaking change of Mocha-v4

[Zhaofeng Miao]

https://travis-ci.org/3rd-Eden/memcached/jobs/324647584

Found that all test passed but the mocha process didn't exit. Mocha 4 or 5 will cause this problem because of this Breaking change

The nature of the problem is that socket didn't shutdown after test case done. We should fix this