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/keepgoing/keepgoing_same_step.sv

@@ -0,0 +1,17 @@
+module test (
+	input clk, a
+);
+	reg [7:0] counter = 0;
+
+	always @(posedge clk) begin
+		counter <= counter + 1;
+	end
+
+	always @(posedge clk) begin
+		if (counter == 3) begin
+			assert(a);
+			assert(!a);
+			assert(0);
+		end
+	end
+endmodule