org-element-clock-parser: Do not be case-sensitive

* lisp/org-element.el (org-element-clock-parser): Fix demanding
upcased "CLOCK:" string.  As per general Org mode syntax principle,
keywords in Org mode are case-insensitive.
* testing/lisp/test-org-element.el (test-org-element/clock-parser):
Add test.

Reported-by: Gregor Zattler <telegraph@gmx.net>
Link: https://orgmode.org/list/875xx8mvvn.fsf@no.lan

lisp/org-element.el

@@ -2291,9 +2291,8 @@ Return a new syntax node of `clock' type containing `:status',
 `:value', `:time', `:begin', `:end', `:post-blank' and
 `:post-affiliated' as properties."
   (save-excursion
-    (let* ((case-fold-search nil)
-	   (begin (point))
-	   (value (progn (search-forward "CLOCK:" (line-end-position) t)
+    (let* ((begin (point))
+	   (value (progn (search-forward "CLOCK:" (line-end-position))
 			 (skip-chars-forward " \t")
 			 (org-element-timestamp-parser)))
 	   (duration (and (search-forward " => " (line-end-position) t)

testing/lisp/test-org-element.el

@@ -1157,6 +1157,15 @@ CLOCK: [2023-10-13 Fri 14:40]--[2023-10-13 Fri 14:51] =>  0:11"
 				  (org-element-property :value clock))
 	    "[2012-01-01 sun. 00:01]"))
     (should-not (org-element-property :duration clock)))
+  ;; clock string should not be case-sensitive.
+  (let ((clock (org-test-with-temp-text "Clock: [2012-01-01 sun. 00:01]"
+		 (org-element-at-point))))
+    (should (eq (org-element-property :status clock) 'running))
+    (should
+     (equal (org-element-property :raw-value
+				  (org-element-property :value clock))
+	    "[2012-01-01 sun. 00:01]"))
+    (should-not (org-element-property :duration clock)))
   ;; Closed clock.
   (let ((clock
 	 (org-test-with-temp-text