fix shouldReplaceLowestPriorityBlockWhenCacheFull flaky test (#1494)

Signed-off-by: Karim TAAM <karim.t2am@gmail.com>

ethereum/eth/src/test/java/org/hyperledger/besu/ethereum/eth/sync/state/cache/PendingBlockCacheTest.java

@@ -134,7 +134,7 @@ public class PendingBlockCacheTest {
 
   private ImmutablePendingBlock generateBlock(final Block parentBlock, final Bytes nodeId) {
     final Block block =
-        gen.block(gen.nextBlockOptions(parentBlock).setTimestamp(System.currentTimeMillis()));
+        gen.block(gen.nextBlockOptions(parentBlock).setTimestamp(System.nanoTime()));
     return ImmutablePendingBlock.builder().block(block).nodeId(nodeId).build();
   }
 }