Added fast tests.

tests/conftest.py

@@ -0,0 +1,17 @@
+#!/usr/bin/python3
+
+import pytest
+
+
+def pytest_addoption(parser):
+    parser.addoption(
+        "--fast", action="store_true", help="Run only fast tests")
+
+
+def pytest_collection_modifyitems(config, items):
+    if config.getoption("--fast"):
+        skip_slow = pytest.mark.skip(reason="Runs only without --fast option.")
+
+        for item in items:
+            if "slow" in item.keywords:
+                item.add_marker(skip_slow)