Add pkcs8_serialization mark
diff --git a/tests/conftest.py b/tests/conftest.py
index 86d5a03..b1326dc 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -18,7 +18,8 @@
 from cryptography.hazmat.backends import _available_backends
 from cryptography.hazmat.backends.interfaces import (
     CMACBackend, CipherBackend, DSABackend, HMACBackend, HashBackend,
-    PBKDF2HMACBackend, RSABackend, TraditionalOpenSSLSerializationBackend
+    PBKDF2HMACBackend, PKCS8SerializationBackend, RSABackend,
+    TraditionalOpenSSLSerializationBackend
 )
 from .utils import check_backend_support, check_for_iface, select_backends
 
@@ -45,6 +46,11 @@
         TraditionalOpenSSLSerializationBackend,
         item
     )
+    check_for_iface(
+        "pkcs8_serialization",
+        PKCS8SerializationBackend,
+        item
+    )
     check_backend_support(item)