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/regression/aim_vs_smt2_nonzero_start_offset.sby

@@ -0,0 +1,36 @@
+[tasks]
+abc_bmc3 bmc
+abc_sim3 bmc
+aiger_avy prove
+aiger_suprove prove
+abc_pdr prove
+
+[options]
+bmc: mode bmc
+prove: mode prove
+expect fail
+wait on
+
+[engines]
+abc_bmc3: abc bmc3
+abc_sim3: abc sim3
+aiger_avy: aiger avy
+aiger_suprove: aiger suprove
+abc_pdr: abc pdr
+
+[script]
+read -sv test.sv
+prep -top test
+
+[file test.sv]
+module test (
+    input clk,
+    input [8:1] nonzero_offset
+);
+    reg [7:0] counter = 0;
+
+    always @(posedge clk) begin
+        if (counter == 3) assert(nonzero_offset[1]);
+        counter <= counter + 1;
+    end
+endmodule