Add max operator to get_relation_last_modified macro (#234)

### Summary

Metadata freshness checks were not working properly if `dbt source
freshness` was ran without using the `loaded_at_field` keyword. This is
because we were not reducing the amount of results to the max
commited_at value in our metadata select statement.

### Description

Added the max operator as suggested by @KingLommel. Also fixed the
relevant test to include more than one snapshot.

### Changelog

-   [x] Added a summary of what this PR accomplishes to CHANGELOG.md

### Related Issue

#229

CHANGELOG.md

@@ -1,6 +1,7 @@
 # dbt-dremio MAIN
 
 -   [#223](https://github.com/dremio/dbt-dremio/issues/224) Implement merge strategy for incremental materializations
+-   [#229](https://github.com/dremio/dbt-dremio/issues/229) Add max operator to get_relation_last_modified macro 
 
 # dbt-dremio v1.7.0
 

dbt/include/dremio/macros/adapters/metadata.sql

@@ -312,7 +312,7 @@ limitations under the License.*/
   {%- if relation.type != 'view' -%}
 
     {%- call statement('last_modified', fetch_result=True) -%}
-          select committed_at as last_modified,
+          select max(committed_at) as last_modified,
                 {{ current_timestamp() }} as snapshotted_at
           from TABLE( table_snapshot('{{relation}}') )
     {%- endcall -%}

tests/functional/adapter/relation/test_get_relation_last_modified.py

@@ -1,6 +1,6 @@
 import pytest
 from dbt.tests.util import run_dbt
-from tests.utils.util import BUCKET
+from tests.utils.util import BUCKET, relation_from_name
 
 
 freshness_via_metadata_schema_yml = """version: 2
@@ -83,7 +83,10 @@ def test_get_last_relation_modified(self, project):
 
         # run command
         result = run_dbt(["seed"])
-
+        relation = relation_from_name(project.adapter, "test_source")
+        result = project.run_sql(
+            f"INSERT INTO {relation} VALUES (10, 'name')", fetch="one"
+        )
         results = run_dbt(["source", "freshness"])
         assert len(results) == 1
         result = results[0]