test: cover the validator, the directory and the site

tests/test_domains.py

@@ -0,0 +1,31 @@
+"""Registrable-domain comparison, which is what the same-site rules rest on."""
+
+import pytest
+
+from mews.fetch import registered_domain, same_site
+
+CASES = [
+    ("example.com", "example.com"),
+    ("img.example.com", "example.com"),
+    ("a.b.example.co.uk", "example.co.uk"),
+    ("EXAMPLE.COM", "example.com"),
+    ("example.com.", "example.com"),
+    ("a.github.io", "a.github.io"),
+    ("not-a-tld", None),
+    ("", None),
+]
+
+
+@pytest.mark.parametrize(("host", "expected"), CASES)
+def test_registrable_domain(host, expected):
+    assert registered_domain(host) == expected
+
+
+def test_subdomains_of_one_domain_are_one_site():
+    assert same_site("example.com", "img.example.com")
+
+
+def test_two_sites_on_a_shared_suffix_are_not_one_site():
+    """The reason this uses a public suffix list rather than counting labels."""
+    assert not same_site("a.github.io", "b.github.io")
+    assert not same_site("example.co.uk", "evil.co.uk")