Tests for --keep-going

This also changes the test Makefile to run `.check.py` files after
running the corresponding `.sby` file to allow more precise testing of
the keep going feature.

tests/keepgoing_multi_step.sv

@@ -0,0 +1,22 @@
+module test (
+	input clk, a
+);
+	reg [7:0] counter = 0;
+
+	always @(posedge clk) begin
+		counter <= counter + 1;
+	end
+
+	always @(posedge clk) begin
+		assert(0);
+		if (counter == 3 || counter == 7) begin
+			assert(a); // step 3,7
+		end
+		if (counter == 5) begin
+			assert(a); // step 5
+		end
+		if (counter == 7) begin
+			assert(a); // step 7
+		end
+	end
+endmodule