Fix a bug in delete-node where the prev pointer wasn't getting set right

Add more unit tests

core/dlists/dlists-tests.factor

@@ -59,3 +59,6 @@ IN: temporary
 [ 0 ] [ <dlist> dlist-length ] unit-test
 [ 1 ] [ <dlist> 1 over push-front dlist-length ] unit-test
 [ 0 ] [ <dlist> 1 over push-front dup pop-front* dlist-length ] unit-test
+
+[ 4 ] [ <dlist> 1 over push-back 2 over push-back 3 over push-back 4 over push-back 5 over push-back [ 3 = ] over delete-node drop dlist-front dlist-node-next dlist-node-next dlist-node-obj ] unit-test
+[ 2 ] [ <dlist> 1 over push-back 2 over push-back 3 over push-back 4 over push-back 5 over push-back [ 3 = ] over delete-node drop dlist-front dlist-node-next dlist-node-next dlist-node-prev dlist-node-obj ] unit-test

core/dlists/dlists.factor

@@ -107,7 +107,8 @@ PRIVATE>
     {
         { [ 2dup >r dlist-front r> = ] [ drop pop-front* ] }
         { [ 2dup >r dlist-back r> = ] [ drop pop-back* ] }
-        { [ t ] [ dup dlist-node-prev swap dlist-node-next set-prev-when
+        { [ t ] [ dup dlist-node-prev over dlist-node-next set-prev-when
+                  dup dlist-node-next swap dlist-node-prev set-next-when
                   dec-length ] }
     } cond ;