MDL-75898 assignfeedback_editpdf: Add randomness to PDF annotations

The behat step "I draw on the pdf" creates the exact same line every
time it is called. This can cause issues in a situation like the following:

- Student uploads PDF
- Teacher annotates page 1
- Student edits submission, adding another PDF
- Teacher annotates page 1 again
- When the student looks at their annotated PDF it appears the same as
  before they added the second PDF (i.e., they don't see extra pages for
  the second PDF)

Exactly why this happens isn't clear, and in practice this probably never
happens because the chances of drawing the exact same line twice are almost
0. However, when testing the functionality added in MDL-45580 it was causing
issues.

mod/assign/feedback/editpdf/tests/behat/behat_assignfeedback_editpdf.php

@@ -56,23 +56,47 @@ public function ghostscript_is_installed() {
      * @When /^I draw on the pdf$/
      */
     public function i_draw_on_the_pdf() {
+        // There appears to be a bug with detecting changes to
+        // annotations. If a PDF is annotated, then the student
+        // updates the submission, if the teacher then draws the
+        // exact same annotations on the new PDF, the readonly
+        // pages are not updated and the student's view of the
+        // annotated PDF still shows the old PDF. So we add some
+        // randomness in this step to ensure the annotations are
+        // different every time.
+        //
+        // This was added to test MDL-75898. See MDL-76659 for
+        // more details about the bug.
+        //
+        // Note - the start, move and end locations must all be different.
+        // If they are the same, it's possible the PDF tool selected does not activate.
+        $startx = 100 + rand(0, 50);
+        $starty = 250 + rand(0, 50);
         $js = ' (function() {
     var instance = M.assignfeedback_editpdf.instance;
-    var event = { clientX: 100, clientY: 250, preventDefault: function() {} };
+    var event = { clientX: ' . $startx . ', clientY: ' . $starty . ', preventDefault: function() {} };
     instance.edit_start(event);
 }()); ';
         $this->execute_script($js);
         sleep(1);
+
+        // Move slightly in one direction.
+        $movex = $startx + 50;
+        $movey = $starty + 30;
         $js = ' (function() {
     var instance = M.assignfeedback_editpdf.instance;
-    var event = { clientX: 150, clientY: 275, preventDefault: function() {} };
+    var event = { clientX: ' . $movex . ', clientY: ' . $movey . ', preventDefault: function() {} };
     instance.edit_move(event);
 }()); ';
         $this->execute_script($js);
         sleep(1);
+
+        // Move a little further to stop.
+        $endx = $movex + 15;
+        $endy = $movey + 15;
         $js = ' (function() {
     var instance = M.assignfeedback_editpdf.instance;
-    var event = { clientX: 200, clientY: 300, preventDefault: function() {} };
+    var event = { clientX: ' . $endx . ', clientY: ' . $endy . ', preventDefault: function() {} };
     instance.edit_end(event);
 }()); ';
         $this->execute_script($js);