Try to be more aggressive when shutting the test pool down (#434)

Test failures on travis often happen when the pool did not shutdown in time and the test frameworks complains about leaked threads.
Try to wait for task termination first before returning control to the test framework.

Author: knutwalker

tests/src/test/java/org/neo4j/graphalgo/core/utils/ParallelUtilTest.java

@@ -269,7 +269,15 @@ private static void withPool(
         } catch (Throwable throwable) {
             throw Exceptions.launderedException(throwable);
         } finally {
+            List<Runnable> unscheduled = pool.shutdownNow();
+            pool.shutdown();
+            try {
+                pool.awaitTermination(1, TimeUnit.MINUTES);
+            } catch (InterruptedException e) {
+                // ok
+            }
             assertTrue(pool.shutdownNow().isEmpty());
+            assertTrue(pool.isTerminated());
         }
     }