[FLINK-1203] Deactivate fork reuse in tests ()

Works around a potential surefire bug where we see confused class cast exceptions

```
java.lang.ClassCastException: org.apache.flink.api.common.operators.base.DeltaIterationBase cannot be cast to org.apache.flink.api.common.operators.base.GroupReduceOperatorBase
	at org.apache.flink.api.scala.operators.translation.DistinctTranslationTest.testCombinable(DistinctTranslationTest.scala:39)
```