test: Fix FATAL: sorry, too many clients already

Dispose the SQLAlchemy engine object after use within
`PostgresConnector`.

target_postgres/connector.py

@@ -180,8 +180,10 @@ def copy_table_structure(
 
     @contextmanager
     def _connect(self) -> t.Iterator[sqlalchemy.engine.Connection]:
-        with self._engine.connect().execution_options() as conn:
+        engine = self._engine
+        with engine.connect().execution_options() as conn:
             yield conn
+        engine.dispose()
 
     def drop_table(
         self, table: sqlalchemy.Table, connection: sqlalchemy.engine.Connection
@@ -800,9 +802,7 @@ def column_exists(  # type: ignore[override]
         )
 
     def __del__(self):
-        """
-        Dispose underlying SQLAlchemy engine object.
-        """
+        """Dispose underlying SQLAlchemy engine object."""
         if self._cached_engine is not None:
             self._cached_engine.dispose()