bpo-30438 Added test confirming failure to untar from readonly directory

[catherinedevlin]

Issue is still open, but this test (once the @unittest.skip is removed) will confirm the fix.

http://bugs.python.org/issue30438

[the-knights-who-say-ni]

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[catherinedevlin]

Hold up - I think I misunderstood the issue. There may not be a bug here after all, or in any case, this is probably not the solution...