[Python] Fix TestDataFormatterSmartArray to work across python versions.

Python 3 default encoding is utf-8, so taking random bytes and
interpreting them as a string might result in invalid unicode sequences.
As the only thing we care about here is that the formatter shows the
elements of the underyling array, relax the string matching (this is
good enough as all the elements are distinct so they resolve to different
strings).

llvm-svn: 356096