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/mixed.v

@@ -0,0 +1,17 @@
+module test (input CP, CN, CX, input A, B, output reg XP, XN, YP, YN);
+	always @* begin
+		assume (A || B);
+		assume (!A || !B);
+		assert (A != B);
+		cover (A);
+		cover (B);
+	end
+	always @(posedge CP)
+		XP <= A;
+	always @(negedge CN)
+		XN <= B;
+	always @(posedge CX)
+		YP <= A;
+	always @(negedge CX)
+		YN <= B;
+endmodule