Change how we represented that a test requires a backend.

This way is more extensible and requires less maintaince
diff --git a/tests/test_utils.py b/tests/test_utils.py
index da3b1a2..6c8d088 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -27,12 +27,12 @@
 import cryptography_vectors
 
 from .utils import (
-    check_backend_support, check_for_iface, der_encode_dsa_signature,
-    load_cryptrec_vectors, load_fips_dsa_key_pair_vectors,
-    load_fips_dsa_sig_vectors, load_fips_ecdsa_key_pair_vectors,
-    load_fips_ecdsa_signing_vectors, load_hash_vectors, load_kasvs_dh_vectors,
-    load_nist_vectors, load_pkcs1_vectors, load_rsa_nist_vectors,
-    load_vectors_from_file, raises_unsupported_algorithm, select_backends
+    check_backend_support, der_encode_dsa_signature, load_cryptrec_vectors,
+    load_fips_dsa_key_pair_vectors, load_fips_dsa_sig_vectors,
+    load_fips_ecdsa_key_pair_vectors, load_fips_ecdsa_signing_vectors,
+    load_hash_vectors, load_kasvs_dh_vectors, load_nist_vectors,
+    load_pkcs1_vectors, load_rsa_nist_vectors, load_vectors_from_file,
+    raises_unsupported_algorithm, select_backends
 )
 
 
@@ -82,19 +82,6 @@
     assert selected_backends == [b1, b2]
 
 
-def test_check_for_iface():
-    item = pretend.stub(keywords=["fake_name"], funcargs={"backend": True})
-    with pytest.raises(pytest.skip.Exception) as exc_info:
-        check_for_iface("fake_name", FakeInterface, item)
-    assert exc_info.value.args[0] == "True backend does not support fake_name"
-
-    item = pretend.stub(
-        keywords=["fake_name"],
-        funcargs={"backend": FakeInterface()}
-    )
-    check_for_iface("fake_name", FakeInterface, item)
-
-
 def test_check_backend_support_skip():
     supported = pretend.stub(
         kwargs={"only_if": lambda backend: False, "skip_message": "Nope"}