getRollingCountThreadsExecuted() always returns 0 in master

`HystrixThreadPoolMetrics.getRollingCountThreadsExecuted()` always returns `0` in `master` (1.4.x) rather than true count. This was working in 1.3.x. 

This pull request is just a failing test case, **not a fix**. Second test fails in `master` but passes in `1.3.x`. I added `HystrixThreadPoolMetricsTest.resetAll()` to public API in order to properly clean up after test.