Fix some backend feature checks in tests (#4931)

* Remove irrelevant DHBackend test conditions

DHBackend provides functions for plain finite-field Diffie-Hellman.
X25519 and X448 are their own algorithms, and Ed25519 and Ed448 aren't
even Diffie-Hellman primitives.

* Add missing backend support checks.

Some new AES and EC tests did not check for whether the corresponding
mode or curve was supported by the backend.

* Add a DummyMode for coverage

1 files changed, 7 insertions, 3 deletions

diff --git a/tests/hazmat/primitives/test_aes.py b/tests/hazmat/primitives/test_aes.py
index 565cc11d..f1d434f1 100644
--- a/tests/hazmat/primitives/test_aes.py
+++ b/tests/hazmat/primitives/test_aes.py
@@ -13,6 +13,7 @@ from cryptography.hazmat.backends.interfaces import CipherBackend
 from cryptography.hazmat.primitives.ciphers import algorithms, base, modes
 
 from .utils import _load_all_params, generate_aead_test, generate_encrypt_test
+from ...doubles import DummyMode
 from ...utils import load_nist_vectors
 
 
@@ -484,14 +485,17 @@ class TestAESModeGCM(object):
         modes.CFB(bytearray(b"\x00" * 16)),
         modes.CFB8(bytearray(b"\x00" * 16)),
         modes.XTS(bytearray(b"\x00" * 16)),
+        # Add a dummy mode for coverage of the cipher_supported check.
+        DummyMode(),
     ]
 )
 @pytest.mark.requires_backend_interface(interface=CipherBackend)
 def test_buffer_protocol_alternate_modes(mode, backend):
     data = bytearray(b"sixteen_byte_msg")
-    cipher = base.Cipher(
-        algorithms.AES(bytearray(os.urandom(32))), mode, backend
-    )
+    key = algorithms.AES(bytearray(os.urandom(32)))
+    if not backend.cipher_supported(key, mode):
+        pytest.skip("AES in {} mode not supported".format(mode.name))
+    cipher = base.Cipher(key, mode, backend)
     enc = cipher.encryptor()
     ct = enc.update(data) + enc.finalize()
     dec = cipher.decryptor()