Reset Executor after each test in ThreadActivityTest

This is related to #1182.

The tests errored on my local machine:
```
java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@5bb7643d rejected from java.util.concurrent.ThreadPoolExecutor@3ac04654[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 5]
	at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.reject(Unknown Source)
	at java.util.concurrent.ThreadPoolExecutor.execute(Unknown Source)
	at java.util.concurrent.AbstractExecutorService.submit(Unknown Source)
	at org.androidannotations.api.BackgroundExecutor.directExecute(BackgroundExecutor.java:103)
	at org.androidannotations.api.BackgroundExecutor.execute(BackgroundExecutor.java:129)
	at org.androidannotations.test15.ThreadActivity_.backgroundThrowException(ThreadActivity_.java:366)
	at org.androidannotations.test15.ThreadActivityTest.propagateExceptionToGlobalExceptionHandler(ThreadActivityTest.java:391)
```

This was the test execution order:
![untitled](https://cloud.githubusercontent.com/assets/480485/4642953/923f7b9c-544a-11e4-980b-4b4fdbdd28a4.png)

@Artyomcool, please look at this if you can.