Fix TestMiniDumpNew.py test for Python 2/3 issue

On Windows, where we use Python 3 for testing, we have to be more explicit about converting between binary and string representations.  I believe this should still work for Python 2, but I don't have a convenient way to try it out.

Differential Revision: https://reviews.llvm.org/D26643

llvm-svn: 286909