support --backend as a pytest flag to limit to one backend for testing
diff --git a/tests/conftest.py b/tests/conftest.py
index 1d9f96e..5cc3042 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -5,7 +5,12 @@
     HMACBackend, CipherBackend, HashBackend
 )
 
-from .utils import check_for_iface, check_backend_support
+from .utils import check_for_iface, check_backend_support, modify_backend_list
+
+
+def pytest_generate_tests(metafunc):
+    name = metafunc.config.getoption("--backend")
+    modify_backend_list(name, _ALL_BACKENDS)
 
 
 @pytest.fixture(params=_ALL_BACKENDS)
@@ -19,3 +24,10 @@
     check_for_iface("cipher", CipherBackend, item)
     check_for_iface("hash", HashBackend, item)
     check_backend_support(item)
+
+
+def pytest_addoption(parser):
+    parser.addoption(
+        "--backend", action="store", metavar="NAME",
+        help="Only run tests matching the backend NAME."
+    )