Add test showing it is impossible to add CERT entry

with the current REST API, which it was proposed to be used in the
mixnet.

tests/test_gnunet.py

@@ -137,3 +137,24 @@ def test_list_gns_record_type():
     assert 'pepfoundation.alice' == r['record_name']
     # Even if there're several records, all should be type A
     assert 'A' == r['data'][0]['record_type']
+
+
+def test_add_cert_entry():
+    """
+    Test that is not possible to add a record type ``CERT`` via REST.
+
+    Even if it is implement in GNS:
+    https://git.gnunet.org/gnunet.git/tree/src/gnsrecord/plugin_gnsrecord_dns.c#n130
+
+    """
+    r = REST_CLIENT.add_namestore_entry(
+        "alice",
+        "pepfoundation",
+        "foo",
+        "CERT"
+    )
+    assert r[0] is False
+    # In gnunet logs it can be seen the line:
+    # ``gnunet-rest-server-12 ERROR Value invalid for record type```
+    # But the REST API only returns this.
+    assert "Internal Server Error" == r[1]