mkcert.py unittest fail when execute run_tests.sh

Bug: 223706851
Test: bash run_tests.sh
Change-Id: I80ca64f71ae23b57c9f64c5e6af66a0d6afb1d21
diff --git a/setup/mkcert_test.py b/setup/mkcert_test.py
index d4edc8d..0489878 100644
--- a/setup/mkcert_test.py
+++ b/setup/mkcert_test.py
@@ -35,8 +35,9 @@
         self.Patch(mkcert, "UnInstall")
         self.Patch(utils, "Popen")
         self.Patch(shutil, "rmtree")
-        self.Patch(os, "stat", return_value=33188)
+        self.Patch(os, "stat")
         self.Patch(os, "chmod")
+        os.stat().st_mode = 33188
         mkcert.Install()
         os.chmod.assert_not_called()
         shutil.rmtree.assert_not_called()
@@ -46,7 +47,7 @@
 
         self.Patch(os.path, "isdir", return_value=True)
         self.Patch(os.path, "exists", return_value=True)
-        self.Patch(os, "stat", return_value=33184)
+        os.stat().st_mode = 33184
         mkcert.Install()
         os.chmod.assert_called_once()
         shutil.rmtree.assert_called_once()