basics and tests done.

tests/conftest.py

@@ -1,10 +1,29 @@
 """
-    Dummy conftest.py for cristallina.
+    conftest.py for cristallina.
 
-    If you don't know what this is for, just leave it empty.
     Read more about conftest.py under:
     - https://docs.pytest.org/en/stable/fixture.html
     - https://docs.pytest.org/en/stable/writing_plugins.html
 """
 
-# import pytest
+import pytest
+
+
+def pytest_addoption(parser):
+    parser.addoption(
+        "--runregression", action="store_true", default=False, help="run slow regression tests"
+    )
+
+
+def pytest_configure(config):
+    config.addinivalue_line("markers", "regression: mark test as a slow regression test to run")
+
+
+def pytest_collection_modifyitems(config, items):
+    if config.getoption("--runregression"):
+        # --runregression given in cli: do not skip slow tests
+        return
+    skip_regression = pytest.mark.skip(reason="need --runregression option to run")
+    for item in items:
+        if "regression" in item.keywords:
+            item.add_marker(skip_regression)