Testcase for exception binhex.Error to increase test coverage.

unittest provides a utility to help with testing that correct exceptions are raised:

http://docs.python.org/dev/library/unittest#unittest.TestCase.assertRaises

The testcase updated accordingly to Nick Coghlan's suggestion to use assertRaises() since the testcase deals with an exception.

The testcase provided shows up a resource leakage:
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C:\_cpython\cpython>PCbuild\python_d.exe  -m test.regrtest test_binhex
[1/1] test_binhex
C:\_cpython\cpython\lib\unittest\case.py:574: ResourceWarning: unclosed file <_io.BufferedWriter name='@test_5592_tmp2'>
  callableObj(*args, **kwargs)
1 test OK.
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The resource leakage occurs in module binhex.py in binhex(inp,out) > BinHex.__init__() > _writeinfo() when the exception binhex.Error is raised in _writeinfo() at the code line:
    raise Error('Filename too long')
The issue is that when the exception is thrown the file is left unclosed.

One of possible fixes is to catch the thrown exception and close the unclosed file in the BinHex.__init__().

Arkady, I don't see why you need to catch exception. Just wrap the _writeinfo call into a try-finally block, and close ofp in the finally.

Well, I believe that in BinHex.__init__() I cannot just wrap the _writeinfo call into a try-finally block, and close ofp in the finally.

I see that in the case of normal operation when the exception is not thrown, the output file descriptor ofp is used in binhex(inp,out) as a result of BinHex.__init__() and so need remain open until binhex(inp,out) close it explicitly.

New changeset 5c2d15c6007e by Nick Coghlan in branch '3.2':
Close #11577: Improve binhex test coverage and fix ResourceWarning
http://hg.python.org/cpython/rev/5c2d15c6007e

This is fixed on default as well, I just stuffed the merge so the history looks odd and the integration script didn't pick it up.

You can see the additional changes I made in the linked changeset:
- try/except with a flag to clean up implicitly created file objects when binhex.__init__ fails
- change the getfileinfo code to use a with statement so it is also more exception tolerant