ox: Fix test errors

* lisp/ox.el (org-export-data-with-backend): Set temporary back-end as
  the new back-end in local communication channel.
(org-export-filter-apply-functions): Handle corner case where back-end
is nil.
* testing/lisp/test-ox.el: Small refactoring.

lisp/ox.el

@@ -2250,9 +2250,10 @@ recursively convert DATA using BACKEND translation table."
    ;; memoization.
    (org-combine-plists
     info
-    (list :translate-alist (org-export-get-all-transcoders backend)
+    (list :back-end backend
+	  :translate-alist (org-export-get-all-transcoders backend)
 	  ;; Size of the hash table is reduced since this function
-	  ;; will probably be used on short trees.
+	  ;; will probably be used on small trees.
 	  :exported-data (make-hash-table :test 'eq :size 401)))))
 
 (defun org-export--interpret-p (blob info)
@@ -2754,7 +2755,8 @@ VALUE is ignored.
 Call is done in a LIFO fashion, to be sure that developer
 specified filters, if any, are called first."
   (catch 'exit
-    (let ((backend-name (org-export-backend-name (plist-get info :back-end))))
+    (let* ((backend (plist-get info :back-end))
+	   (backend-name (and backend (org-export-backend-name backend))))
       (dolist (filter filters value)
 	(let ((result (funcall filter value backend-name info)))
 	  (cond ((not result) value)

testing/lisp/test-ox.el

@@ -2246,51 +2246,48 @@ Another text. (ref:text)
 
 (ert-deftest test-org-export/table-cell-alignment ()
   "Test `org-export-table-cell-alignment' specifications."
-  (let ((org-table-number-fraction 0.5)
-	(org-table-number-regexp "^[0-9]+$"))
-    ;; 1. Alignment is primarily determined by alignment cookies.
-    (org-test-with-temp-text "| <l> | <c> | <r> |"
-      (let* ((tree (org-element-parse-buffer))
-	     (info `(:parse-tree ,tree)))
-	(should
-	 (equal
-	  '(left center right)
-	  (mapcar (lambda (cell) (org-export-table-cell-alignment cell info))
-		  (org-element-map tree 'table-cell 'identity))))))
-    ;; 2. The last alignment cookie has precedence.
-    (org-test-with-parsed-data "
+  ;; 1. Alignment is primarily determined by alignment cookies.
+  (should
+   (equal '(left center right)
+	  (let ((org-table-number-fraction 0.5)
+		(org-table-number-regexp "^[0-9]+$"))
+	    (org-test-with-parsed-data "| <l> | <c> | <r> |"
+	      (mapcar (lambda (cell)
+			(org-export-table-cell-alignment cell info))
+		      (org-element-map tree 'table-cell 'identity))))))
+  ;; 2. The last alignment cookie has precedence.
+  (should
+   (equal '(right right right)
+	  (org-test-with-parsed-data "
 | <l8> |
 | cell |
 | <r9> |"
-      (should
-       (equal
-	'(right right right)
-	(mapcar (lambda (cell) (org-export-table-cell-alignment cell info))
-		(org-element-map tree 'table-cell 'identity)))))
-    ;; 3. If there's no cookie, cell's contents determine alignment.
-    ;;    A column mostly made of cells containing numbers will align
-    ;;    its cells to the right.
-    (org-test-with-parsed-data "
+	    (mapcar (lambda (cell) (org-export-table-cell-alignment cell info))
+		    (org-element-map tree 'table-cell 'identity)))))
+  ;; 3. If there's no cookie, cell's contents determine alignment.
+  ;;    A column mostly made of cells containing numbers will align
+  ;;    its cells to the right.
+  (should
+   (equal '(right right right)
+	  (let ((org-table-number-fraction 0.5)
+		(org-table-number-regexp "^[0-9]+$"))
+	    (org-test-with-parsed-data "
 | 123       |
 | some text |
 | 12345     |"
-      (should
-       (equal
-	'(right right right)
-	(mapcar (lambda (cell)
-		  (org-export-table-cell-alignment cell info))
-		(org-element-map tree 'table-cell 'identity)))))
-    ;; 4. Otherwise, they will be aligned to the left.
-    (org-test-with-parsed-data "
+	      (mapcar (lambda (cell)
+			(org-export-table-cell-alignment cell info))
+		      (org-element-map tree 'table-cell 'identity))))))
+  ;; 4. Otherwise, they will be aligned to the left.
+  (should
+   (equal '(left left left)
+	  (org-test-with-parsed-data "
 | text      |
 | some text |
 | \alpha    |"
-      (should
-       (equal
-	'(left left left)
-	(mapcar (lambda (cell)
-		  (org-export-table-cell-alignment cell info))
-		(org-element-map tree 'table-cell 'identity)))))))
+	    (mapcar (lambda (cell)
+		      (org-export-table-cell-alignment cell info))
+		    (org-element-map tree 'table-cell 'identity info))))))
 
 (ert-deftest test-org-export/table-cell-borders ()
   "Test `org-export-table-cell-borders' specifications."