Add mul_unsigned test

tests/xilinx/mul_unsigned.v

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+/*
+Example from: https://www.xilinx.com/support/documentation/sw_manuals/xilinx2019_1/ug901-vivado-synthesis.pdf [p. 89].
+*/
+
+// Unsigned 16x24-bit Multiplier
+// 1 latency stage on operands
+// 3 latency stage after the multiplication
+// File: multipliers2.v
+//
+module mul_unsigned (clk, A, B, RES);
+parameter WIDTHA = /*16*/ 6;
+parameter WIDTHB = /*24*/ 9;
+input clk;
+input [WIDTHA-1:0] A;
+input [WIDTHB-1:0] B;
+output [WIDTHA+WIDTHB-1:0] RES;
+reg [WIDTHA-1:0] rA;
+reg [WIDTHB-1:0] rB;
+reg [WIDTHA+WIDTHB-1:0] M [3:0];
+integer i;
+always @(posedge clk)
+ begin
+ rA <= A;
+ rB <= B;
+ M[0] <= rA * rB;
+ for (i = 0; i < 3; i = i+1)
+ M[i+1] <= M[i];
+ end
+assign RES = M[3];
+endmodule

tests/xilinx/mul_unsigned.ys

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+read_verilog mul_unsigned.v
+proc
+hierarchy -top mul_unsigned
+equiv_opt -assert -map +/xilinx/cells_sim.v synth_xilinx # equivalency check
+design -load postopt # load the post-opt design (otherwise equiv_opt loads the pre-opt design)
+cd mul_unsigned # Constrain all select calls below inside the top module
+stat
+select -assert-count 1 t:BUFG
+select -assert-count 1 t:DSP48E1
+select -assert-count 15 t:SRL16E
+select -assert-none t:DSP48E1 t:SRL16E t:BUFG %% t:* %D