don't check writability in test for get_home_dir when HOME is undefined

Some pathological environments such as gentoo sandboxes can give invalid results.

closes  #1998

IPython/utils/tests/test_path.py

@@ -159,9 +159,8 b' def test_get_home_dir_4():'
     """get_home_dir() still works if $HOME is not set"""
 
     if 'HOME' in env: del env['HOME']
-    # this should still succeed, but we don't know what the answer should be
-    home = path.get_home_dir(True)
-    nt.assert_true(path._writable_dir(home))
+    # this should still succeed, but we don't care what the answer is
+    home = path.get_home_dir(False)
 
 @with_environment
 def test_get_home_dir_5():