Fix index names in MySQL comparison files

When MySQL creates an implicit index for a FK, the index name matches
the FK constraint name. With the auto-generated FK naming change,
indexes are now named 'articles_user_id' instead of 'user_id'.

Author: dereuromark

tests/comparisons/Diff/default/schema-dump-test_comparisons_mysql.lock

@@ -20,7 +20,7 @@ public function up(): void
             ->removeIndexByName('UNIQUE_SLUG')
             ->removeIndexByName('rating_index')
             ->removeIndexByName('BY_NAME')
-            ->removeIndexByName('user_id')
+            ->removeIndexByName('articles_user_id')
             ->update();
 
         $this->table('articles')
@@ -215,7 +215,7 @@ public function down(): void
             )
             ->addIndex(
                 $this->index('user_id')
-                    ->setName('user_id')
+                    ->setName('articles_user_id')
             )
             ->update();