Add unit tests for exclusive_primary_alt_value_setting method

I debated about parametrizing these tests, and it can be done. However,
I found that the resulting code ended up being about the same number of
lines and slightly less readable (in my opinion). Given the simplicity of
these tests, I think not parametrizing them is okay.

tests/unit/config/test_utils.py

@@ -0,0 +1,41 @@
+import pytest
+
+from dbt.config.utils import exclusive_primary_alt_value_setting
+from dbt.exceptions import DbtWarnErrorOptionsError
+
+
+class TestExclusivePrimaryAltValueSetting:
+    @pytest.fixture(scope="class")
+    def primary_key(self) -> str:
+        return "key_a"
+
+    @pytest.fixture(scope="class")
+    def alt_key(self) -> str:
+        return "key_b"
+
+    @pytest.fixture(scope="class")
+    def value(self) -> str:
+        return "I LIKE CATS"
+
+    def test_primary_set(self, primary_key: str, alt_key: str, value: str):
+        test_dict = {primary_key: value}
+        exclusive_primary_alt_value_setting(test_dict, primary_key, alt_key)
+        assert test_dict.get(primary_key) == value
+        assert test_dict.get(alt_key) is None
+
+    def test_alt_set(self, primary_key: str, alt_key: str, value: str):
+        test_dict = {alt_key: value}
+        exclusive_primary_alt_value_setting(test_dict, primary_key, alt_key)
+        assert test_dict.get(primary_key) == value
+        assert test_dict.get(alt_key) == value
+
+    def test_primary_and_alt_set(self, primary_key: str, alt_key: str, value: str):
+        test_dict = {primary_key: value, alt_key: value}
+        with pytest.raises(DbtWarnErrorOptionsError):
+            exclusive_primary_alt_value_setting(test_dict, primary_key, alt_key)
+
+    def test_neither_primary_nor_alt_set(self, primary_key: str, alt_key: str):
+        test_dict = {}
+        exclusive_primary_alt_value_setting(test_dict, primary_key, alt_key)
+        assert test_dict.get(primary_key) is None
+        assert test_dict.get(alt_key) is None