org-table-sort-lines: Fix case-sensitive sorting

* org-table.el (org-table-sort-lines): Fix case sensitive sorting,
improve docstring.
* test-org-table.el (test-org-table/sort-lines): Enforce C locale when
testing alphabetic sorting.

‘sort-subr’ ignores ‘sort-fold-case’ when a predicate is provided. To
correctly handle case-sensitivity, we now bake it into the predicate.

Since we are now sorting according to the user’s locale, WITH-CASE
will not make a difference in most instances, since most locales
always sort case-insensitively (cf. how GNU sort ignores the ‘-f’
switch).  We now mention this in the function docstring.

In order to meaningfully test case-sensitive sorting, we now enforce
the C locale in the respective unit test.

lisp/org-table.el

@@ -1714,7 +1714,8 @@ alphabetically, numerically, or by time (as given in a time stamp
 in the field, or as a HH:MM value).  Sorting in reverse order is
 also possible.
 
-With prefix argument WITH-CASE, alphabetic sorting will be case-sensitive.
+With prefix argument WITH-CASE, alphabetic sorting will be case-sensitive
+if the locale allows for it.
 
 If SORTING-TYPE is specified when this function is called from a Lisp
 program, no prompting will take place.  SORTING-TYPE must be a character,
@@ -1766,8 +1767,7 @@ function is being called interactively."
       ;; Determine arguments for `sort-subr'.  Also record original
       ;; position.  `org-table-save-field' cannot help here since
       ;; sorting is too much destructive.
-      (let* ((sort-fold-case (not with-case))
-	     (coordinates
+      (let* ((coordinates
 	      (cons (count-lines (point-min) (line-beginning-position))
 		    (current-column)))
 	     (extract-key-from-field
@@ -1794,7 +1794,8 @@ function is being called interactively."
 	     (predicate
 	      (cl-case sorting-type
 		((?n ?N ?t ?T) #'<)
-		((?a ?A) #'org-string-collate-lessp)
+		((?a ?A) (if with-case #'org-string-collate-lessp
+			   (lambda (s1 s2) (org-string-collate-lessp s1 s2 nil t))))
 		((?f ?F)
 		 (or compare-func
 		     (and interactive?

testing/lisp/test-org-table.el

@@ -1689,28 +1689,33 @@ See also `test-org-table/copy-field'."
 	  (org-test-with-temp-text "| <point>1 | 2 |\n| 5 | 3 |\n| 2 | 4 |\n"
 	    (org-table-sort-lines nil ?N)
 	    (buffer-string))))
-  ;; Sort alphabetically.
-  (should
-   (equal "| a | x |\n| B | 4 |\n| c | 3 |\n"
-	  (org-test-with-temp-text "| <point>a | x |\n| c | 3 |\n| B | 4 |\n"
-	    (org-table-sort-lines nil ?a)
-	    (buffer-string))))
-  (should
-   (equal "| c | 3 |\n| B | 4 |\n| a | x |\n"
-	  (org-test-with-temp-text "| <point>a | x |\n| c | 3 |\n| B | 4 |\n"
-	    (org-table-sort-lines nil ?A)
-	    (buffer-string))))
-  ;; Sort alphabetically with case.
-  (should
-   (equal "| C |\n| a |\n| b |\n"
-	  (org-test-with-temp-text "| <point>a |\n| C |\n| b |\n"
-	    (org-table-sort-lines t ?a)
-	    (buffer-string))))
-  (should
-   (equal "| C |\n| b |\n| a |\n"
-	  (org-test-with-temp-text "| <point>a |\n| C |\n| b |\n"
-	    (org-table-sort-lines nil ?A)
-	    (buffer-string))))
+  ;; Sort alphabetically.  Enforce the C locale for consistent results.
+  (let ((original-string-collate-lessp (symbol-function 'string-collate-lessp)))
+    (cl-letf (((symbol-function 'string-collate-lessp)
+	       (lambda (s1 s2 &optional locale ignore-case)
+		 (funcall original-string-collate-lessp
+			  s1 s2 "C" ignore-case))))
+      (should
+       (equal "| a | x |\n| B | 4 |\n| c | 3 |\n"
+	      (org-test-with-temp-text "| <point>a | x |\n| c | 3 |\n| B | 4 |\n"
+				       (org-table-sort-lines nil ?a)
+				       (buffer-string))))
+      (should
+       (equal "| c | 3 |\n| B | 4 |\n| a | x |\n"
+	      (org-test-with-temp-text "| <point>a | x |\n| c | 3 |\n| B | 4 |\n"
+				       (org-table-sort-lines nil ?A)
+				       (buffer-string))))
+      ;; Sort alphabetically with case.
+      (should
+       (equal "| C |\n| a |\n| b |\n"
+	      (org-test-with-temp-text "| <point>a |\n| C |\n| b |\n"
+				       (org-table-sort-lines t ?a)
+				       (buffer-string))))
+      (should
+       (equal "| C |\n| b |\n| a |\n"
+	      (org-test-with-temp-text "| <point>a |\n| C |\n| b |\n"
+				       (org-table-sort-lines nil ?A)
+				       (buffer-string))))))
   ;; Sort by time (timestamps)
   (should
    (equal