refactored append and char test

tests/append.c

@@ -0,0 +1,17 @@
+#include "../main.h"
+#include <stdio.h>
+
+/**
+ *main - ensure append work
+ */
+int main(void)
+{
+	char *str, c;
+
+	str = "hello";
+	c = 'c';
+	str = append(str, c);
+	_printf("%s\n", str);
+	return (0);
+
+}