Add tests for EAN checksum calculation

Closes: #215

Author: WhyNotHugo

tests/test_ean.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+from barcode.ean import EAN13
+
+
+def test_ean_checksum() -> None:
+    ean = EAN13("842169142322")  # input has 12 digits
+    assert ean.calculate_checksum() == 0
+
+    ean = EAN13("8421691423220")  # input has 13 digits
+    assert ean.calculate_checksum() == 0