Stop using unibyte buffers for ert backtraces

* lisp/emacs-lisp/ert.el
(ert-results-pop-to-backtrace-for-test-at-point):
Set multibyte true, not false.  This copies a
debugger-setup-buffer change from 2009-08-30, and stops the
"Backtrace for" header line containing ^X and ^Y.

lisp/emacs-lisp/ert.el

@@ -2406,8 +2406,7 @@ To be used in the ERT results buffer."
            (buffer-disable-undo)
            (erase-buffer)
            (ert-simple-view-mode)
-           ;; Use unibyte because `debugger-setup-buffer' also does so.
-           (set-buffer-multibyte nil)
+           (set-buffer-multibyte t)     ; mimic debugger-setup-buffer
            (setq truncate-lines t)
            (ert--print-backtrace backtrace t)
            (goto-char (point-min))