MDL-70829 block_myoverview: Mark up decorative image properly

* The zero state image is purely decorative so there's no need to
set a non-empty alt text for it.
* The presentation role is also unnecessary for decorative image with
empty alt.
* Accessibility Behat tests added to cover changes

blocks/myoverview/templates/zero-state.mustache

@@ -51,8 +51,7 @@
         <div class="text-xs-center text-center mt-3" data-region="empty-message">
             <img class="empty-placeholder-image-lg mt-1"
                     src="{{nocoursesimg}}"
-                    alt="{{{ title }}}"
-                    role="presentation"
+                    alt=""
             >
             <h5 class="h5 mt-3 mb-0">{{{ title }}}</h5>
             <p class="mt-3 mb-0">{{{ intro }}}</p>

blocks/myoverview/tests/behat/block_myoverview_zerostate.feature

@@ -70,3 +70,8 @@ Feature: Zero state on my overview block
     And "Create course" "button" should exist
     And I click on "Create course" "button"
     And I should see "Add a new course"
+
+  @accessibility
+  Scenario: Evaluate the accessibility of the My courses (zero state)
+    When I am on the "My courses" page logged in as "manager"
+    Then the page should meet accessibility standards