Add basic test for _deep_update

tests/unit/test_config.py

@@ -0,0 +1,17 @@
+import pytest
+
+from esmvalcore._config._config import _deep_update
+
+TEST_DEEP_UPDATE = [([{}], {}), ([dict(a=1, b=2), dict(a=3)], dict(a=3, b=2)),
+                    ([
+                        dict(a=dict(b=1, c=dict(d=2)), e=dict(f=4, g=5)),
+                        dict(a=dict(b=2, c=3))
+                    ], dict(a=dict(b=2, c=3), e=dict(f=4, g=5)))]
+
+
+@pytest.mark.parametrize('dictionaries, expected_merged', TEST_DEEP_UPDATE)
+def test_deep_update(dictionaries, expected_merged):
+    merged = dictionaries[0]
+    for update in dictionaries[1:]:
+        merged = _deep_update(merged, update)
+    assert expected_merged == merged