pp-fill: Fix tests breakage

pp-fill: Fix tests breakage

* lisp/emacs-lisp/pp.el (pp-to-string, pp-buffer, pp): Preserve old
behavior of (almost always) returning a trailing newline.

* test/lisp/emacs-lisp/pp-tests.el (pp-print-quote): Adjust tests, now
that `pp-to-string` always returns a trailing newline, rather than only
most of the time.

* test/lisp/emacs-lisp/backtrace-tests.el
(backtrace-tests--single-and-multi-line): Make the test less sensitive
to the choice of what is "pretty".