test-org.el: Actually test org-copy-visible

* testing/lisp/test-org.el (test-org/copy-visible): Switch defun to
ert-deftest so that the test is actually executed, and resolve
failures in test.

Make two adjustments so that the now executed test passes:

  - Correct the end value of the hidden part of the string for one
    test case.

  - Disable org-unfontify-region so that the invisible properties show
    up in the temporary buffer.

    I think org-unfontify-region should probably be changed to _not_
    remove the invisible property, but it's a longstanding behavior.
    Changing it would need more thought and isn't something that
    should be done on the bugfix branch.

testing/lisp/test-org.el

@@ -8176,8 +8176,12 @@ CLOSED: %s
      (org-show-set-visibility 'minimal)
      (org-invisible-p2))))
 
-(defun test-org/copy-visible ()
+(ert-deftest test-org/copy-visible ()
   "Test `org-copy-visible' specifications."
+  ;;`org-unfontify-region', which is wired up to
+  ;; `font-lock-unfontify-region-function', removes the invisible text
+  ;; property, among other things.
+  (cl-letf (((symbol-function 'org-unfontify-region) #'ignore))
     (should
      (equal "Foo"
 	    (org-test-with-temp-text "Foo"
@@ -8187,7 +8191,7 @@ CLOSED: %s
     ;; Skip invisible characters by text property.
     (should
      (equal "Foo"
-	  (org-test-with-temp-text #("F<hidden>oo" 1 7 (invisible t))
+	    (org-test-with-temp-text #("F<hidden>oo" 1 9 (invisible t))
 	      (let ((kill-ring nil))
 	        (org-copy-visible (point-min) (point-max))
 	        (current-kill 0 t)))))
@@ -8221,7 +8225,7 @@ CLOSED: %s
 	        #("aXbXc" 1 2 (invisible t) 3 4 (invisible t))
 	      (let ((kill-ring nil))
 	        (org-copy-visible (point-min) (point-max))
-	      (current-kill 0 t))))))
+	        (current-kill 0 t)))))))
 
 (ert-deftest test-org/set-visibility-according-to-property ()
   "Test `org-set-visibility-according-to-property' specifications."