MAINT: mock slowest test.

The slowest test of our CI is ~3sec as it use subprocess os/system.
Mocking make it instantaneous.

With our big elements matrix in CI that should save us a bunch of time
in total from PR submission to green.

IPython/core/tests/test_interactiveshell.py

@@ -17,6 +17,7 @@
 import sys
 import tempfile
 import unittest
+import pytest
 from unittest import mock
 
 from os.path import join
@@ -635,10 +636,20 @@ def test_control_c(self, *mocks):
             )
         self.assertEqual(ip.user_ns["_exit_code"], -signal.SIGINT)
 
-    def test_magic_warnings(self):
-        for magic_cmd in ("pip", "conda", "cd"):
-            with self.assertWarnsRegex(Warning, "You executed the system command"):
-                ip.system_raw(magic_cmd)
+
+@pytest.mark.parametrize("magic_cmd", ["pip", "conda", "cd"])
+def test_magic_warnings(magic_cmd):
+    if sys.platform == "win32":
+        to_mock = "os.system"
+    else:
+        to_mock = "subprocess.call"
+    with mock.patch(to_mock, return_value=0) as mock_sub:
+        with pytest.warns(Warning, match=r"You executed the system command"):
+            ip.system_raw(magic_cmd)
+        mock_sub.assert_called_once_with(
+            magic_cmd, shell=True, executable=os.environ.get("SHELL", None)
+        )
+
 
 # TODO: Exit codes are currently ignored on Windows.
 class TestSystemPipedExitCode(ExitCodeChecks):