fix mock issue in pool test

fix mock issue in pool test

Due to the change for #5648 in ad14471bc99c2fb2315d3333a95dd,
the mock in test_recycle_pool_no_race needs to implement
an empty handle_error dispatcher.

the exception was in a thread so did not interrupt the
test suite:

File "/home/classic/dev/sqlalchemy/lib/sqlalchemy/engine/base.py", line 2059, in _handle_dbapi_exception
      for fn in self.dialect.dispatch.handle_error:
  TypeError: 'Mock' object is not iterable

Change-Id: I764357c48aa1bf53a572d8ee24c89a7463505092