Test for (* keep *)-ed abc9_box_id

tests/simple_abc9/abc9.v

@@ -291,3 +291,19 @@ module abc9_test035(input clk, d, output reg [1:0] q);
 always @(posedge clk) q[0] <= d;
 always @(negedge clk) q[1] <= q[0];
 endmodule
+
+module abc9_test036(input A, B, S, output [1:0] O);
+  (* keep *)
+  MUXF8 m  (
+    .I0(I0),
+    .I1(I1),
+    .O(O[0]),
+    .S(S)
+  );
+  MUXF8 m2  (
+    .I0(I0),
+    .I1(I1),
+    .O(O[1]),
+    .S(S)
+  );
+endmodule