MDL-45296 tests: ensure all submissions exist

The changes performed with the change from
create_from_user() to create_from_submission()
in practice enforce a new restriction about
submissions having to exist in order to fire
their lock/unlock events.

This did not exist before the change and, also,
it seems that the assign api itself, submission->lock() ...
also accepts non existing submissions.

So I was not 100% sure about how to fix these events:

1) enforcing submission to exist.
2) firing them only if the submission exist.

I've gone with 1) for now, making tests to pass. But will
raise the question in the Tracker, just in case we have
to move to the 2) approach for any reason.

mod/assign/tests/events_test.php

@@ -212,6 +212,7 @@ public function test_submission_locked() {
         $assign = $this->create_instance();
         $sink = $this->redirectEvents();
 
+        $submission = $assign->get_user_submission($this->students[0]->id, true);
         $assign->lock_submission($this->students[0]->id);
 
         $events = $sink->get_events();
@@ -446,6 +447,7 @@ public function test_submission_unlocked() {
         $assign = $this->create_instance();
         $sink = $this->redirectEvents();
 
+        $submission = $assign->get_user_submission($this->students[0]->id, true);
         $assign->unlock_submission($this->students[0]->id);
 
         $events = $sink->get_events();

mod/assign/tests/externallib_test.php

@@ -470,7 +470,7 @@ public function test_lock_submissions() {
                                               $course->id,
                                               $teacherrole->id);
 
-        // Create a student1 with an online text submission.
+        // Create a student1 and student2 with an online text submission.
         // Simulate a submission.
         $this->setUser($student1);
         $submission = $assign->get_user_submission($student1->id, true);
@@ -481,6 +481,15 @@ public function test_lock_submissions() {
         $plugin = $assign->get_submission_plugin_by_type('onlinetext');
         $plugin->save($submission, $data);
 
+        $this->setUser($student2);
+        $submission = $assign->get_user_submission($student2->id, true);
+        $data = new stdClass();
+        $data->onlinetext_editor = array('itemid'=>file_get_unused_draft_itemid(),
+                                         'text'=>'Submission text',
+                                         'format'=>FORMAT_MOODLE);
+        $plugin = $assign->get_submission_plugin_by_type('onlinetext');
+        $plugin->save($submission, $data);
+
         // Ready to test.
         $this->setUser($teacher);
         $students = array($student1->id, $student2->id);
@@ -534,7 +543,7 @@ public function test_unlock_submissions() {
                                               $course->id,
                                               $teacherrole->id);
 
-        // Create a student1 with an online text submission.
+        // Create a student1 and student2 with an online text submission.
         // Simulate a submission.
         $this->setUser($student1);
         $submission = $assign->get_user_submission($student1->id, true);
@@ -545,6 +554,15 @@ public function test_unlock_submissions() {
         $plugin = $assign->get_submission_plugin_by_type('onlinetext');
         $plugin->save($submission, $data);
 
+        $this->setUser($student2);
+        $submission = $assign->get_user_submission($student2->id, true);
+        $data = new stdClass();
+        $data->onlinetext_editor = array('itemid'=>file_get_unused_draft_itemid(),
+                                         'text'=>'Submission text',
+                                         'format'=>FORMAT_MOODLE);
+        $plugin = $assign->get_submission_plugin_by_type('onlinetext');
+        $plugin->save($submission, $data);
+
         // Ready to test.
         $this->setUser($teacher);
         $students = array($student1->id, $student2->id);