test/support.py: remove TESTFN if it is a directory

Because of my previous commit (r81171), test_os failed without removing TESTFN
directory (shutil.rmtree() was broken). Some buildbots still have a @test
directory and some tests fail because of that.

The bug is reproductible with:

  mkdir @test
  touch @test/abc
  ./python Lib/test/regrtest.py test_site
1 file changed