[3.12] gh-115538: Use isolate mode when running venv test_multiproces… (#117264)

[3.12] gh-115538: Use isolate mode when running venv test_multiprocessing_recursion() (GH-117116)
(cherry picked from commit 4ec3477)

Co-authored-by: Victor Stinner <[email protected]>

Author: aisk

Lib/test/test_venv.py

@@ -494,7 +494,7 @@ def test_multiprocessing_recursion(self):
         envpy = os.path.join(os.path.realpath(self.env_dir),
                              self.bindir, self.exe)
         script = os.path.join(TEST_HOME_DIR, '_test_venv_multiprocessing.py')
-        subprocess.check_call([envpy, script])
+        subprocess.check_call([envpy, "-I", script])
 
     @unittest.skipIf(os.name == 'nt', 'not relevant on Windows')
     def test_deactivate_with_strict_bash_opts(self):