test: Fix FATAL: sorry, too many clients already

Within `PostgresConnector(SQLConnector)`, this aims to dispose the
underlying SQLAlchemy engine object when being torn down.

Author: amotl

target_postgres/connector.py

@@ -799,6 +799,13 @@ def column_exists(  # type: ignore[override]
             schema_name=schema_name, table_name=table_name, connection=connection
         )
 
+    def __del__(self):
+        """
+        Dispose underlying SQLAlchemy engine object.
+        """
+        if self._cached_engine is not None:
+            self._cached_engine.dispose()
+
 
 class NOTYPE(TypeDecorator):
     """Type to use when none is provided in the schema."""