[py3] Make this test compatible with Python 3 where bytes and
strings don't mix so easily. This fixes the last remaining failure
I have in 'check-all' on a system with both Python3 and and Python2
installed.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@224947 91177308-0d34-0410-b5e6-96231b3b80d8