Add a test for "consolidate timestamp macros". (#200)

### Description

Adds a test for "consolidate timestamp macros" at dbt-labs/dbt-spark#480.

tests/functional/adapter/utils/test_timestamps.py

@@ -0,0 +1,18 @@
+import pytest
+from dbt.tests.adapter.utils.test_timestamps import BaseCurrentTimestamps
+
+
+class TestCurrentTimestampDatabricks(BaseCurrentTimestamps):
+    @pytest.fixture(scope="class")
+    def models(self):
+        return {
+            "get_current_timestamp.sql": "select {{ current_timestamp() }} as current_timestamp"
+        }
+
+    @pytest.fixture(scope="class")
+    def expected_schema(self):
+        return {"current_timestamp": "timestamp"}
+
+    @pytest.fixture(scope="class")
+    def expected_sql(self):
+        return """select current_timestamp() as current_timestamp"""