Fix ftp test_ls_root_dircache()

[Astropilot]

Oops, following the tests results for #1643 I noticed this warning:

fsspec/implementations/tests/test_ftp.py::test_ls_root_dircache
  /home/runner/micromamba/envs/test_env/lib/python3.8/site-packages/_pytest/unraisableexception.py:80: PytestUnraisableExceptionWarning: Exception ignored in: <function FTPFileSystem.__del__ at 0x7fee3a7bb9d0>
  
  Traceback (most recent call last):
    File "/home/runner/work/filesystem_spec/filesystem_spec/fsspec/implementations/ftp.py", line 251, in __del__
      self.ftp.close()
  AttributeError: 'NoneType' object has no attribute 'close'
  
    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

I was not aware (but it makes sense) of the closing connection with the __del__ method.
Fixed it by restoring the ftp variable after test usage.

[martindurant]

OK :).