Fixes compile error in Eclipse in ClusterRerouteRequestTests (#21400)

Before this change Eclipse (4.6.1) would show compile errors in `ClusterRerouteRequestTests` for the elements in `RANDOM_COMMAND_GENERATORS`. This seems to be because the eclipse compiler does not recognised that the elements in the list are all `Supplier`s of bjects that are subclasses of `AllocationCommand`.

This change fixes the problem by adding a generics hint to the `Arrays.toList()` call.

core/src/test/java/org/elasticsearch/action/admin/cluster/reroute/ClusterRerouteRequestTests.java

@@ -58,7 +58,8 @@ import static org.elasticsearch.common.unit.TimeValue.timeValueMillis;
  */
 public class ClusterRerouteRequestTests extends ESTestCase {
     private static final int ROUNDS = 30;
-    private final List<Supplier<AllocationCommand>> RANDOM_COMMAND_GENERATORS = unmodifiableList(Arrays.asList(
+    private final List<Supplier<AllocationCommand>> RANDOM_COMMAND_GENERATORS = unmodifiableList(
+            Arrays.<Supplier<AllocationCommand>> asList(
             () -> new AllocateReplicaAllocationCommand(randomAsciiOfLengthBetween(2, 10), between(0, 1000),
                     randomAsciiOfLengthBetween(2, 10)),
             () -> new AllocateEmptyPrimaryAllocationCommand(randomAsciiOfLengthBetween(2, 10), between(0, 1000),