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/support.py

@@ -0,0 +1,69 @@
+"""Support code for test_*.py files"""
+# Author: Collin Winter
+
+# Python imports
+import unittest
+import os
+import os.path
+from textwrap import dedent
+
+# Local imports
+import lib2to3
+from lib2to3 import pytree, refactor
+from lib2to3.pgen2 import driver as pgen2_driver
+
+lib2to3_dir = os.path.dirname(lib2to3.__file__)
+test_dir = os.path.dirname(__file__)
+proj_dir = os.path.normpath(os.path.join(test_dir, ".."))
+grammar_path = os.path.join(lib2to3_dir, "Grammar.txt")
+grammar = pgen2_driver.load_grammar(grammar_path)
+grammar_no_print_statement = pgen2_driver.load_grammar(grammar_path)
+del grammar_no_print_statement.keywords["print"]
+driver = pgen2_driver.Driver(grammar, convert=pytree.convert)
+driver_no_print_statement = pgen2_driver.Driver(
+    grammar_no_print_statement,
+    convert=pytree.convert
+)
+
+def parse_string(string):
+    return driver.parse_string(reformat(string), debug=True)
+
+def run_all_tests(test_mod=None, tests=None):
+    if tests is None:
+        tests = unittest.TestLoader().loadTestsFromModule(test_mod)
+    unittest.TextTestRunner(verbosity=2).run(tests)
+
+def reformat(string):
+    return dedent(string) + "\n\n"
+
+def get_refactorer(fixer_pkg="lib2to3", fixers=None, options=None):
+    """
+    A convenience function for creating a RefactoringTool for tests.
+
+    fixers is a list of fixers for the RefactoringTool to use. By default
+    "lib2to3.fixes.*" is used. options is an optional dictionary of options to
+    be passed to the RefactoringTool.
+    """
+    if fixers is not None:
+        fixers = [fixer_pkg + ".fixes.fix_" + fix for fix in fixers]
+    else:
+        fixers = refactor.get_fixers_from_package(fixer_pkg + ".fixes")
+    options = options or {}
+    return refactor.RefactoringTool(fixers, options, explicit=True)
+
+def _all_project_files(root, files):
+    for dirpath, dirnames, filenames in os.walk(root):
+        for filename in filenames:
+            if not filename.endswith(".py"):
+                continue
+            files.append(os.path.join(dirpath, filename))
+
+def all_project_files():
+    files = []
+    _all_project_files(lib2to3_dir, files)
+    _all_project_files(test_dir, files)
+    # Sort to get more reproducible tests
+    files.sort()
+    return files
+
+TestCase = unittest.TestCase