test: Use Database.open instead of Database.new

- Follow standard pattern used throughout test suite
- Database.open creates in-memory database by default

Author: suketa

test/duckdb_test/table_function_test.rb

@@ -48,7 +48,7 @@ def _test_register_without_callbacks
     private
 
     def setup_incomplete_function
-      database = DuckDB::Database.new
+      database = DuckDB::Database.open
       conn = database.connect
       table_function = DuckDB::TableFunction.new
       table_function.name = 'incomplete_function'