Update readme, add success message at end of tests

README.md

@@ -117,7 +117,6 @@ To do
 - ls
 - make-prime-list
 - md5sum
-- mkdir
 - mkfifo
 - mknod
 - mktemp

cat/test.rs

@@ -4,6 +4,7 @@ fn main() {
     test_output_multi_files_print_all_chars();
     test_stdin_squeeze();
     test_stdin_number_non_blank();
+    println("cat tests completed successfully!\n");
 }
 
 fn test_output_multi_files_print_all_chars() {

mkdir/test.rs

@@ -11,6 +11,7 @@ fn main() {
     test_mkdir_mode();
     test_mkdir_parent();
     test_mkdir_no_parent();
+    println("mkdir tests completed successfully!\n");
 }
 
 fn cleanup() {