ytdl-sub: disable test_directory_exists

`test_directory_exists` assumes the root directory is not writeable, which is host dependent.
In my case it fails with nix-portable in bwrap mode, despite me having removed the write-bit in the root root bindmount.

Full error:

```
ytdl-sub> ____________________ TestFileHandler.test_directory_exists _____________________
ytdl-sub>
ytdl-sub> self = <tests.unit.utils.test_file_handler.TestFileHandler object at 0x7ffff2bc07d0>
ytdl-sub>
ytdl-sub> .    def test_directory_exists(self):
ytdl-sub>         if IS_WINDOWS:
ytdl-sub>             return
ytdl-sub>
ytdl-sub>         assert FileHandler.is_path_writable("/tmp")
ytdl-sub>         assert FileHandler.is_path_writable("/tmp/")
ytdl-sub>         assert FileHandler.is_path_writable("/tmp/non-existent")
ytdl-sub>         assert FileHandler.is_path_writable("/tmp/non-existent/")
ytdl-sub>         assert FileHandler.is_path_writable("/tmp/non-existent/nested")
ytdl-sub>
ytdl-sub> >       assert not FileHandler.is_path_writable("/lol-in-root")
ytdl-sub> E       AssertionError: assert not True
ytdl-sub> E        +  where True = is_path_writable('/lol-in-root')
ytdl-sub> E        +    where is_path_writable = FileHandler.is_path_writable
ytdl-sub>
ytdl-sub> tests/unit/utils/test_file_handler.py:16: AssertionError
```

pkgs/by-name/yt/ytdl-sub/package.nix

@@ -59,6 +59,8 @@ python3Packages.buildPythonApplication (finalAttrs: {
     "test_no_config_works"
     "test_presets_run"
     "test_thumbnail"
+    # fails in bwrap nix-portable sandbox
+    "test_directory_exists"
   ];
 
   disabledTestPaths = [