Add counter.sv SVA test

tests/sva/counter.sv

@@ -0,0 +1,29 @@
+module top (input clk, reset, up, down, output reg [7:0] cnt);
+	always @(posedge clk) begin
+		if (reset)
+			cnt <= 0;
+		else if (up)
+			cnt <= cnt + 1;
+		else if (down)
+			cnt <= cnt - 1;
+	end
+
+	default clocking @(posedge clk); endclocking
+	default disable iff (reset);
+
+	assert property (up |=> cnt == $past(cnt) + 8'd1);
+	// assert property (up [*2] |=> cnt == $past(cnt, 2) + 8'd2);
+	// assert property (up ##1 up |=> cnt == $past(cnt, 2) + 8'd2);
+
+`ifndef FAIL
+	assume property (down |-> !up);
+`endif
+	assert property (down |=> cnt == $past(cnt) - 8'd1);
+
+	property down_n(n);
+		down [*n] |=> cnt == $past(cnt, n) + n;
+	endproperty
+
+	// assert property (down_n(3));
+	// assert property (down_n(5));
+endmodule