we should use der rather than ber

Doesn't make a difference in this specific case but it is correct.
diff --git a/tests/utils.py b/tests/utils.py
index c80e97b..5c0e234 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -18,7 +18,7 @@
 import re
 from contextlib import contextmanager
 
-from pyasn1.codec.ber import encoder
+from pyasn1.codec.der import encoder
 from pyasn1.type import namedtype, univ
 
 import pytest