Add the missing test case for the previous patch

lisp/dabbrev.el (dabbrev--substitute-expansion): Return EXPANSION after
any processing.
lisp/dabbrev.el (dabbrev-expand): Set EXPANSION to the return value of
DABBREV--SUBSTITUTE-EXPANSION.
test/automated/dabbrev-tests.el (dabbrev-expand-test): Test for bug#1948.

test/automated/dabbrev-tests.el

@@ -0,0 +1,42 @@
+;;; dabbrev-tests.el --- Test suite for dabbrev.
+
+;; Copyright (C) 2016 Free Software Foundation, Inc.
+
+;; Author: Alan Third <alan@idiocy.org>
+;; Keywords: dabbrev
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert)
+(require 'dabbrev)
+
+(ert-deftest dabbrev-expand-test ()
+  "Test for bug#1948.
+When DABBREV-ELIMINATE-NEWLINES is non-nil (the default),
+repeated calls to DABBREV-EXPAND can result in the source of
+first expansion being replaced rather than the destination."
+  (with-temp-buffer
+   (insert "ab  x\na\nab  y")
+   (goto-char 8)
+   (save-window-excursion
+     (set-window-buffer nil (current-buffer))
+     ;; M-/ SPC M-/ M-/
+     (execute-kbd-macro "\257 \257\257"))
+   (should (string= (buffer-string) "ab  x\nab y\nab  y"))))