fix test directory migration (#27885)

* fix test directory migration

* change assert to self.assertXXX

python/paddle/fluid/tests/unittests/test_directory_migration.py

@@ -94,8 +94,8 @@ class TestDirectory(unittest.TestCase):
             stderr=subprocess.PIPE)
         stdout, stderr = ps_proc.communicate()
 
-        assert "Error" not in str(stderr), "Error: Can't" \
-            " import Module {}".format(module)
+        self.assertFalse("Error" in str(stderr),
+                         "ErrorMessage:\n{}".format(bytes.decode(stderr)))
 
     def test_old_directory(self):
         old_directory = [
@@ -177,7 +177,7 @@ if count != {len_old_directory}:
             stderr=subprocess.PIPE)
         stdout, stderr = ps_proc.communicate()
 
-        assert "Error" not in str(stdout), str(stdout)
+        self.assertFalse("Error" in str(stdout), bytes.decode(stdout))
 
 
 if __name__ == '__main__':