[3.13] gh-144601: Use _testmultiphase instead of _testsinglephase in test_importlib (GH-144769) (GH-144877)

[3.14] gh-144601: Use `_testmultiphase` instead of `_testsinglephase` in `test_importlib` (GH-144769)
(cherry picked from commit d148662)

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>

Author: miss-islington

Lib/test/test_importlib/util.py

@@ -16,7 +16,7 @@
 import tempfile
 import types
 
-_testsinglephase = import_helper.import_module("_testsinglephase")
+import_helper.import_module("_testmultiphase")
 
 
 BUILTINS = types.SimpleNamespace()