gh-126014: test_makefile_test_folders: Ignore basically-empty directories

The code in test_makefile was attempting to ignore any any
non-interesting files, but missed some corners:

1. There is never a *file* called `__pycache__`.
2. A directory containing only a `__pycache__` subdirectory should be
   ignored.
3. A directory containing only hidden files should be ignored.

Simplify this all into a couple of filters that let us check for empty
lists.

Author: stefanor

Lib/test/test_tools/test_makefile.py

@@ -48,15 +48,18 @@ def test_makefile_test_folders(self):
             if dirname == '__pycache__' or dirname.startswith('.'):
                 dirs.clear()  # do not process subfolders
                 continue
-            # Skip empty dirs:
+
+            # Skip empty dirs (ignoring hidden files and __pycache__):
+            files = [
+                filename for filename in files
+                if not filename.startswith('.')
+            ]
+            dirs = [
+                dirname for dirname in dirs
+                if not dirname.startswith('.') and dirname != "__pycache__"
+            ]
             if not dirs and not files:
                 continue
-            # Skip dirs with hidden-only files:
-            if files and all(
-                filename.startswith('.') or filename == '__pycache__'
-                for filename in files
-            ):
-                continue
 
             relpath = os.path.relpath(dirpath, support.STDLIB_DIR)
             with self.subTest(relpath=relpath):