[3.13] gh-124842: Fix test.support.import_helper.make_legacy_pyc() (GH-124843) (GH-124853)

For source file "path/to/file.py" it created file with incorrect path
"/absolute/path/to/path/to/file.pyc" instead of "path/to/file.pyc".
(cherry picked from commit 60ff67d)

Co-authored-by: Serhiy Storchaka <[email protected]>

Author: miss-islington

Lib/test/support/import_helper.py

@@ -58,8 +58,8 @@ def make_legacy_pyc(source):
     :return: The file system path to the legacy pyc file.
     """
     pyc_file = importlib.util.cache_from_source(source)
-    up_one = os.path.dirname(os.path.abspath(source))
-    legacy_pyc = os.path.join(up_one, source + 'c')
+    assert source.endswith('.py')
+    legacy_pyc = source + 'c'
     shutil.move(pyc_file, legacy_pyc)
     return legacy_pyc