add demux and testbench

gates/demux.v

@@ -0,0 +1,9 @@
+module demux(y, z, a, sel);
+    input a, sel;
+    output y, z;
+    wire notSel;
+
+    not(notSel, sel);
+    and(z, a, sel);
+    and(y, a, notSel);
+endmodule

gates/testbench/demux._tb.v

@@ -0,0 +1,18 @@
+module demux_tb();
+    reg a,b, sel;
+    output y;
+    demux dut(y, z, a, sel);
+        initial begin
+            a = 0; sel = 0;
+            $monitor("IN: %b ",a, "SEL: ",sel, " OUT: a: %b, b: %b", y, z);
+            #5;
+            a = 1;sel = 0;
+            $monitor("IN: %b ",a, "SEL: ",sel, " OUT: a: %b, b: %b", y, z);
+            #5;
+            a = 0; sel = 1;
+            $monitor("IN: %b ",a, "SEL: ",sel, " OUT: a: %b, b: %b", y, z);
+            #5;
+            a = 1; sel = 1;
+            $monitor("IN: %b ",a, "SEL: ",sel, " OUT: a: %b, b: %b", y, z);
+        end
+endmodule