reorganize and add support for multiple testbenches

tb/.gitignore

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+**/*.out
+**/*.vcd

tb/top/tb_top.gtkw

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+[*]
+[*] GTKWave Analyzer v3.3.86 (w)1999-2017 BSI
+[*] Fri Jul  2 07:11:25 2021
+[*]
+[dumpfile] "/home/brendan/Documents/projects/ice40/tb/top/tb_top.vcd"
+[dumpfile_mtime] "Fri Jul  2 07:10:41 2021"
+[dumpfile_size] 14906
+[savefile] "/home/brendan/Documents/projects/ice40/tb/top/tb_top.gtkw"
+[timestart] 0
+[size] 1920 1052
+[pos] -29 -29
+*-20.000000 739000 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
+[sst_width] 223
+[signals_width] 94
+[sst_expanded] 1
+[sst_vpaned_height] 311
+@28
+tb_top.clk
+tb_top.reset
+@8023
+tb_top.led[7:0]
+[pattern_trace] 1
+[pattern_trace] 0

tb/top/tb_top.v

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+// Author:  Brendan Haines
+// Date:    2021-07-02
+
+`timescale 1ns/1ps
+
+module tb_top();
+
+reg clk, reset;
+wire [7:0] led;
+
+top dut(
+    .clk(clk),
+    .n_reset(~reset),
+    .led(led)
+);
+
+always #5 clk = ~clk;
+
+initial begin
+    $dumpfile("tb_top.vcd");
+    $dumpvars(0, tb_top);
+
+    clk = 0;
+    reset = 1;
+
+    #10
+    reset = 0;
+
+    #2560
+    // #2560
+    $display("----------");
+    $display("Finished simulation.");
+    $display("Simulation time:\t%d ns", $realtime);
+    $finish;
+end
+
+endmodule