bpo-30818: test_ftplib calls asyncore.close_all() (#2514)

Always clear asyncore socket map using
asyncore.close_all(ignore_all=True) in tearDown() method.

This change should fix this warning:

Warning -- asyncore.socket_map was modified by test_ftplib
  Before: {}
  After:  {4: <test.test_ftplib.DummyTLS_FTPServer 127.0.0.1:0 at 0x805feccf0>}