Add test, fix issue raised by test

python/plugins/processing/tests/testdata/qgis_algorithm_tests1.yaml

@@ -154,7 +154,7 @@ tests:
           precision: 0
         }, {
           input: 'intval',
-          aggregate: 'first_value',
+          aggregate: 'last_value',
           delimiter: '',
           name: 'intval',
           type: 2,

src/analysis/processing/qgsalgorithmaggregate.cpp

@@ -183,6 +183,7 @@ QVariantMap QgsAggregateAlgorithm::processAlgorithm( const QVariantMap &paramete
       groupSinks.emplace_back( std::move( sink ) );
       group.layer = layer;
       group.firstFeature = feature;
+      group.lastFeature = feature;
       groups[key] = group;
       keys.append( key );
     }