CH Preparation sometimes not complete for tests
I've stumbled over this problem only in the tests so far and it happens very rarely (maybe only on slow virtual computers?):

```
java.lang.AssertionError: Properties for fastest_mtb should NOT contain error CH preparation incomplete [5]
    at com.graphhopper.GraphHopperTest.testMultipleCHPreparationsInParallel(GraphHopperTest.java:894)
```
