Fix test breakage in test-markdown-subtree/move-up.

The test is broken prior to Emacs 24.5 due to Emacs bug #19102:
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19102
emacs-mirror/emacs@b3910f

While this isn't a bug in the latest version of Emacs 24, it does impact
the version of Emacs 24 in e.g. the most recent long-term-support
distribution of Ubuntu (14.04; the emacs package for that provides Emacs
24.3).

Stephen Berman notes in the bug report (message jrblevin#14):

> The error occurs when the sexp `(= (char-after) ?\n)' in
> outline-move-subtree-down is evaluated at eob. This can happen either
> when calling outline-move-subtree-down on either of the last two
> subtrees, or when calling outline-move-subtree-up on the last subtree.

Failed to find an easy way to work around that in
markdown-mode-move-subtree-up, so this does not change that function.
Instead, it adjusts the test to avoid triggering that outline-mode bug.
It also changes the assertion about the test-markdown-subtree/move-up
to assert the contents of the error message instead of asserting that
the type of the error is 'user-error. That's because prior to the change
that fixed this bug in Emacs 24.5, the type of that error message was
simply 'error.

tests/markdown-test.el

@@ -1468,12 +1468,25 @@ the opening bracket of [^2], and then subsequent functions would kill [^2])."
 
 (ert-deftest test-markdown-subtree/move-up ()
   "Test `markdown-move-subtree-up'."
-  (markdown-test-string "# 1 #\n\n## 1.1 ##\n\n### 1.1.1 ###\n\n## 1.2 ##\n\n### 1.2.1 ###\n\n# 2 #\n"
+  ;; Note that prior to Emacs 24.5, this does not work for the last subtree in
+  ;; the buffer due to Emacs bug #19102:
+  ;; https://debbugs.gnu.org/cgi/bugreport.cgi?bug=19102
+  ;; https://github.com/emacs-mirror/emacs/commit/b3910f
+  ;; That also corrects the type of the "Cannot move pase superior level" error
+  ;; from 'error to 'user-error.
+  (markdown-test-string "# 1 #\n\n## 1.1 ##\n\n### 1.1.1 ###\n\n## 1.2 ##\n\n### 1.2.1 ###\n\n# 2 #\n# Extra\n"
                         (re-search-forward "^# 2")
                         (markdown-move-subtree-up)
-                        (should (string-equal (buffer-string) "# 2 #\n# 1 #\n\n## 1.1 ##\n\n### 1.1.1 ###\n\n## 1.2 ##\n\n### 1.2.1 ###\n\n"))
-                        ;; Second attempt should fail, leaving buffer unchanged
-                        (should-error (markdown-move-subtree-up) :type 'user-error)))
+                        (should (string-equal (buffer-string) "# 2 #\n# 1 #\n\n## 1.1 ##\n\n### 1.1.1 ###\n\n## 1.2 ##\n\n### 1.2.1 ###\n\n# Extra\n"))
+                        ;; Second attempt should fail, leaving buffer unchanged.
+                        ;; (This way of asserting the contents of the error
+                        ;; message is a bit convoluted and more fragile than
+                        ;; ideal. But prior to Emacs 24.5, the type of this
+                        ;; error is just 'error, and a bare "should-error" is
+                        ;; really overly broad.)
+                        (should (string-equal
+                                 "Cannot move past superior level"
+                                 (second (should-error (markdown-move-subtree-up)))))))
 
 (ert-deftest test-markdown-subtree/move-down ()
   "Test `markdown-move-subtree-down'."