ComputeNodeTest : Ensure edit thread stops before test finishes
ComputeNodeTest : Ensure edit thread stops before test finishes

Otherwise we get nodes being deleted on a background thread _after_ the test has finished. This was causing intermittent failures in subsequent tests - one example of which is below :

```
FAIL: testDurations (GafferTest.PerformanceMonitorTest.PerformanceMonitorTest)

----------------------------------------------------------------------

Traceback (most recent call last):

File "python/GafferTest/PerformanceMonitorTest.py", line 235, in testDurations

self.assertEqual( m.plugStatistics( n1["out"] ).hashCount, 1 )

AssertionError: 2 != 1
```

In this case the deletion of plugs on the background thread was flushing the hash cache mid-test, leading to more calls to `hash()` than were expected.

andrewkaufman/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,hradec/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,GafferHQ/gaffer,boberfly/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,hradec/gaffer,lucienfostier/gaffer,boberfly/gaffer,ivanimanishi/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,boberfly/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,hradec/gaffer,ivanimanishi/gaffer,ivanimanishi/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,GafferHQ/gaffer,johnhaddon/gaffer,ImageEngine/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,ivanimanishi/gaffer,boberfly/gaffer,hradec/gaffer