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

This refines the fix for bug#72561: commit 0a50019308 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.

lisp/emacs-lisp/ert.el

@@ -1316,13 +1316,9 @@ empty string."
 (defun ert--pp-with-indentation-and-newline (object)
   "Pretty-print OBJECT, indenting it to the current column of point.
 Ensures a final newline is inserted."
-  (let ((begin (point))
-        (cols (current-column))
-        (pp-escape-newlines t)
+  (let ((pp-escape-newlines t)
         (print-escape-control-characters t))
-    (pp object (current-buffer))
-    (unless (bolp) (insert "\n"))
-    (indent-rigidly begin (point) cols)))
+    (pp object (current-buffer))))
 
 (defun ert--insert-infos (result)
   "Insert `ert-info' infos from RESULT into current buffer.