Add tests for utf8_step_forward

I am not hooking these to "make test", for two reasons:

1. utf8_step_forward is inside #ifdef CONFIG_UTF8 and I don't see
   how to make tests conditional on such options.

2. test/libtest.sh was copied from Git, which is under GPLv2-only.
   Adding more dependencies on it could make ELinks more difficult
   to relicense under GPLv2-or-later.

src/intl/test/test-utf8-step-forward

@@ -0,0 +1,11 @@
+#! /bin/sh -e
+
+./utf8-step-test forward cells-more 999 日本人 9 6
+./utf8-step-test forward cells-more 6 日本人 9 6
+./utf8-step-test forward cells-more 5 日本人 9 6
+./utf8-step-test forward cells-more 4 日本人 6 4
+
+./utf8-step-test forward cells-fewer 999 日本人 9 6
+./utf8-step-test forward cells-fewer 6 日本人 9 6
+./utf8-step-test forward cells-fewer 5 日本人 6 4
+./utf8-step-test forward cells-fewer 4 日本人 6 4

src/intl/test/utf8-step-test.c

@@ -0,0 +1,85 @@
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "elinks.h"
+
+#include "intl/charsets.h"
+#include "util/test.h"
+
+enum direction {
+	DIRECTION_FORWARD,
+	DIRECTION_BACKWARD
+};
+
+static enum direction
+parse_direction(const char *name)
+{
+	if (strcmp(name, "forward") == 0)
+		return DIRECTION_FORWARD;
+	else if (strcmp(name, "backward") == 0)
+		return DIRECTION_BACKWARD;
+	else
+		die("direction must be forward or backward, not %s\n", name);
+}
+
+static enum utf8_step
+parse_way(const char *name)
+{
+	if (strcmp(name, "characters") == 0)
+		return UTF8_STEP_CHARACTERS;
+	else if (strcmp(name, "cells-fewer") == 0)
+		return UTF8_STEP_CELLS_FEWER;
+	else if (strcmp(name, "cells-more") == 0)
+		return UTF8_STEP_CELLS_MORE;
+	else
+		die("way must be characters, cells-fewer, or cells-more; not %s\n", name);
+}
+
+int
+main(int argc, char **argv)
+{
+	enum direction direction;
+	enum utf8_step way;
+	int max_steps;
+	unsigned char *begin, *end;
+	int expect_bytes;
+	int expect_steps;
+	int bytes;
+	int steps;
+
+	if (argc != 7)
+		die("usage: %s (forward|backward) (characters|cells-fewer|cells-more) \\\n"
+		    "\tMAX-STEPS STRING EXPECT-BYTES EXPECT-STEPS\n",
+		    argc ? argv[0] : "utf8-step-test");
+
+	direction = parse_direction(argv[1]);
+	way = parse_way(argv[2]);
+	max_steps = atoi(argv[3]);
+	begin = argv[4];
+	end = begin + strlen(begin);
+	expect_bytes = atoi(argv[5]);
+	expect_steps = atoi(argv[6]);
+
+	switch (direction) {
+	case DIRECTION_FORWARD:
+		bytes = utf8_step_forward(begin, end, max_steps, way, &steps) - begin;
+		break;
+	case DIRECTION_BACKWARD:
+		bytes = end - utf8_step_backward(end, begin, max_steps, way, &steps);
+		break;
+	default:
+		abort();
+	}
+
+	if (bytes != expect_bytes || steps != expect_steps)
+		die("FAIL: bytes=%d (expected %d); steps=%d (expected %d)\n",
+		    bytes, expect_bytes, steps, expect_steps);
+
+	return 0;
+}