Refactor tests

Organize tests into subdirectories and use a new makefile that scans
.sby files and allows selecting tests by mode, engine, solver and/or
subdirectory. Automatically skips tests that use engines/solvers that
are not found in the PATH.

See `cd tests; make help` for a description of supported make targets.

tests/unsorted/both_ex.v

@@ -0,0 +1,25 @@
+module test(
+  input clk,
+  input [7:0] data
+  );
+
+localparam MAX_COUNT = 8'd111;
+reg [7:0] count = 8'd0;
+reg [7:0] margin = MAX_COUNT;
+
+always @ (posedge clk) begin
+  if (data > margin) begin
+    count <= 8'd0;
+    margin <= MAX_COUNT;
+  end else begin
+    count <= count + data;
+    margin <= margin - data;
+  end
+
+  assume (data < 8'd40);
+  assert (count <= MAX_COUNT);
+  cover (count == 8'd42);
+  cover (count == 8'd111);
+end
+
+endmodule