(pp): Indent lines right when starting in col > 0

(pp): Indent lines right when starting in col > 0

This refines the fix for bug#72561: commit 0a500193087e fixes
the bug by changing `ert--pp-with-indentation-and-newline`,
but it turns out that `pp` was inconsistent (it sometimes
indented the subsequent lines correctly and sometimes not,
depending on the current-buffer's major mode).
So the fix really should be in `pp`, which is what this patch does.

* lisp/emacs-lisp/pp.el (pp): Appropriately indent subsequent lines
if the first line is not inserted in column 0 (tho only when
printing into a buffer since otherwise it's somewhere between
ill-defined and impossible to implement).
* lisp/emacs-lisp/ert.el (ert--pp-with-indentation-and-newline):
Remove the indentation code after `pp` which was just working
around the bug in `pp`.  Also remove the redundant addition of
`\n` since `pp` always does it nowadays.
* test/lisp/help-mode-tests.el (help-mode-tests-xref-on-pp):
Fix thinko.

(cherry picked from commit d3311499339fab0371cb7502a1e2481fbcd2c65d)