Separate tests in test_uu leak files of depend on the file leaked in other tests.

$ ./python -m test -m test_decode test_uu 
...
Warning -- files was modified by test_uu
  Before: []
  After:  ['@test_12637_tmpi'] 
test_uu failed (env changed)

$ ./python -m test -m test_decode_filename test_uu
...
Warning -- files was modified by test_uu
  Before: []
  After:  ['@test_12627_tmpi'] 
test_uu failed (env changed)

$ ./python -m test -m test_decodetwice test_uu
...
test test_uu failed -- Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/test/test_uu.py", line 263, in test_decodetwice
    f = open(self.tmpin, 'rb')
FileNotFoundError: [Errno 2] No such file or directory: '@test_12622_tmpi'