Add unit test for newly added Mergeable class

tests/unit/test_contracts_util.py

@@ -0,0 +1,26 @@
+import unittest
+
+from dataclasses import dataclass
+from dbt_common.contracts.util import Mergeable
+from typing import List, Optional
+
+
+@dataclass
+class ExampleMergableClass(Mergeable):
+    attr_a: str
+    attr_b: Optional[int]
+    attr_c: Optional[List[str]]
+
+
+class TestMergableClass(unittest.TestCase):
+    def test_mergeability(self):
+        mergeable1 = ExampleMergableClass(
+            attr_a="loses", attr_b=None, attr_c=["I'll", "still", "exist"]
+        )
+        mergeable2 = ExampleMergableClass(attr_a="Wins", attr_b=1, attr_c=None)
+        merge_result: ExampleMergableClass = mergeable1.merged(mergeable2)
+        assert (
+            merge_result.attr_a == mergeable2.attr_a
+        )  # mergeable2's attr_a is the "last" non None value
+        assert merge_result.attr_b == mergeable2.attr_b  # mergeable1's attrb_b value was None
+        assert merge_result.attr_c == mergeable1.attr_c  # mergeable2's attr_c value was None