[lit] Support parsing scripts with inconsistent or invalid encodings.

 - For whatever reason, we have a lot of test files with bogus unicode
   characters. This patch allows those scripts to still be parsed on Python3 by
   changing the parsing logic to work on binary files, and only require the
   actual script commands to be convertible to ascii.

 - This patch has been tweaked to now ensure that the command strings are not of
   unicode type on Python 2.6-7.