ls: C sort order and fix subdirectory listing

[tertsdiepraam]

Fixes #2149

• The sort order is reverted to a simple string compare without any preprocessing (lowercasing & ignoring leading periods) to match the C locale. This makes the uutils ls faster than GNU on my machine, but this is misleading because GNU does localization by default. The BENCHMARKING.md therefore includes a note stating that comparisons should be done with LC_ALL=C.
• The name of the . folder is fixed and tests are added for this case.

Before

❯ target/release/coreutils ls -al test_folder/
drwxr-xr-x  3 terts terts 4096 May  1 09:16 test_folder
drwxr-xr-x 15 terts terts 4096 May  2 10:15 ..
-rw-r--r--  1 terts terts    0 Apr 30 21:23 Formula
-rw-r--r--  1 terts terts    0 May  1 09:16 formula
drwxr-xr-x  2 terts terts 4096 Apr 30 21:23 .git
-rw-r--r--  1 terts terts    0 Apr 30 21:23 Makefile
-rw-r--r--  1 terts terts    0 Apr 30 21:23 README.md

After

❯ target/release/coreutils ls -al test_folder/
drwxr-xr-x  3 terts terts 4096 May  1 09:16 .
drwxr-xr-x 15 terts terts 4096 May  2 10:15 ..
drwxr-xr-x  2 terts terts 4096 Apr 30 21:23 .git
-rw-r--r--  1 terts terts    0 Apr 30 21:23 Formula
-rw-r--r--  1 terts terts    0 Apr 30 21:23 Makefile
-rw-r--r--  1 terts terts    0 Apr 30 21:23 README.md
-rw-r--r--  1 terts terts    0 May  1 09:16 formula

[sylvestre]

Fails on windows:

---- test_ls::test_ls_a stdout ----
current_directory_resolved: 
touch: C:\Users\RUNNER~1\AppData\Local\Temp\.tmp4rYDF8\.test-1
mkdir: C:\Users\RUNNER~1\AppData\Local\Temp\.tmp4rYDF8\some-dir
touch: C:\Users\RUNNER~1\AppData\Local\Temp\.tmp4rYDF8\some-dir\.test-2
run: D:\a\coreutils\coreutils\target\x86_64-pc-windows-gnu\debug\coreutils.exe ls
run: D:\a\coreutils\coreutils\target\x86_64-pc-windows-gnu\debug\coreutils.exe ls -a
thread 'test_ls::test_ls_a' panicked at 'Stdout does not match regex:
.  ..  .test-1  some-dir
', tests\common\util.rs:319:13


failures:
    test_ls::test_ls_a

[tertsdiepraam]

Should be fixed now!