run-tests: use byte strings for inserted output
We were inserting str on Python 3 which resulted in mixed
str/bytes types on the list. This would later blow up when
trying to write str to the .err file opened in bytes mode.
Differential Revision: https://phab.mercurial-scm.org/D7222
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors