gh-54781: Move Lib/lib2to3/tests/ to Lib/test/test_lib2to3/ (#94049)

* Move Lib/lib2to3/tests/ to Lib/test/test_lib2to3/.
* Remove Lib/test/test_lib2to3.py.
* Update imports.
* all_project_files(): use different paths and sort files
  to make the tests more reproducible.
* Update references to tests.

Lib/test/test_lib2to3/test_all_fixers.py

@@ -0,0 +1,41 @@
+"""Tests that run all fixer modules over an input stream.
+
+This has been broken out into its own test module because of its
+running time.
+"""
+# Author: Collin Winter
+
+# Python imports
+import os.path
+import test.support
+import unittest
+
+# Local imports
+from . import support
+
+
+@test.support.requires_resource('cpu')
+class Test_all(support.TestCase):
+
+    def setUp(self):
+        self.refactor = support.get_refactorer()
+
+    def refactor_file(self, filepath):
+        if test.support.verbose:
+            print(f"Refactor file: {filepath}")
+        if os.path.basename(filepath) == 'infinite_recursion.py':
+            # bpo-46542: Processing infinite_recursion.py can crash Python
+            # if Python is built in debug mode: lower the recursion limit
+            # to prevent a crash.
+            with test.support.infinite_recursion(150):
+                self.refactor.refactor_file(filepath)
+        else:
+            self.refactor.refactor_file(filepath)
+
+    def test_all_project_files(self):
+        for filepath in support.all_project_files():
+            with self.subTest(filepath=filepath):
+                self.refactor_file(filepath)
+
+if __name__ == '__main__':
+    unittest.main()