MDL-76733 mod_assign: Address random failure

This random failure was happening because of the time required to render
and interact with the file picker which sometimes exceeded 2 seconds,
depending on the speed of the machine running the tests.

Converting these tests to use an onlinetext submission and drop the JS
requirement means that they are much less susceptible to runtime issues,
and not susceptible to those caused by JS at all.

mod/assign/tests/behat/set_availability.feature

@@ -184,30 +184,29 @@ Feature: Set availability dates for an assignment
     And I should see "No submission" in the "Student 1" "table_row"
     And I should see "Assignment is overdue by: 2 days 5 hours" in the "Student 1" "table_row"
 
-  @javascript @_file_upload
-  Scenario: Late submission will be calculated only when the student starts the assignm
+  Scenario: Late submission will be calculated only when the student starts the assignments
+    # Note: This test has the potential to randomly fail on slower machines.
+    # The timelimit needs to be sufficient to allow the page to load and be interacted with completely.
     Given the following config values are set as admin:
       | config          | value | plugin |
       | enabletimelimit | 1     | assign |
     And the following "activity" exists:
-      | activity                           | assign          |
-      | course                             | C1              |
-      | name                               | Assignment name |
-      | assignsubmission_file_enabled      | 1               |
-      | assignsubmission_file_maxfiles     | 1               |
-      | assignsubmission_file_maxsizebytes | 0               |
-      | submissiondrafts                   | 0               |
-      | timelimit                          | 2               |
-      | allowsubmissionsfromdate_enabled   | 0               |
-      | duedate_enabled                    | 0               |
-      | cutoffdate_enabled                 | 0               |
-      | gradingduedate_enabled             | 0               |
+      | activity                            | assign          |
+      | course                              | C1              |
+      | name                                | Assignment name |
+      | assignsubmission_onlinetext_enabled | 1               |
+      | assignsubmission_file_enabled       | 0               |
+      | submissiondrafts                    | 0               |
+      | timelimit                           | 2               |
+      | allowsubmissionsfromdate_enabled    | 0               |
+      | duedate_enabled                     | 0               |
+      | cutoffdate_enabled                  | 0               |
+      | gradingduedate_enabled              | 0               |
 
     When I am on the "Assignment name" Activity page logged in as student1
     And I wait "3" seconds
     And I click on "Begin assignment" "link"
-    And I click on "Begin assignment" "button"
-    And I upload "lib/tests/fixtures/empty.txt" file to "File submissions" filemanager
+    And I set the field "Online text" to "This is my submission"
     And I press "Save changes"
     Then I should see "Submitted for grading" in the "Submission status" "table_row"
     And I should see "under the time limit" in the "Time remaining" "table_row"