tests: handle string escaping/encoding on Python 3
This code was failing on Python 3 for a few reasons:
1) sys.argv is str and str doesn't have a .decode()
2) the "string_escape" encoding was renamed to "unicode_escape"
It is wonky casting to bytes to str to bytes. But this is test
code, so meh. I don't believe we exercise any code paths in these
tests where the arguments aren't ascii.
Differential Revision: https://phab.mercurial-scm.org/D5667
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors