[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: ZeroIntensity

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()