MDL-70829 block_recentlyaccesseditems: 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/recentlyaccesseditems/templates/no-items.mustache

@@ -27,9 +27,8 @@
 <div class="text-xs-center text-center mt-4" data-region="empty-message">
     <img
         src="{{noitemsimgurl}}"
-        alt="{{#str}} noitems, block_recentlyaccesseditems {{/str}}"
-        role="presentation"
+        alt=""
         style="height: 70px; width: 70px;"
     >
     <p class="text-muted mt-3 mb-0">{{#str}} noitems, block_recentlyaccesseditems {{/str}}</p>
-</div>
+</div>

blocks/recentlyaccesseditems/tests/behat/block_recentlyaccesseditems_dashboard.feature

@@ -23,15 +23,19 @@ Feature: The recently accessed items block allows users to easily access their m
       | name     | Test forum name |
     And I log in as "student1"
 
+  @accessibility
   Scenario: User has not accessed any item
     Then I should see "No recent items" in the "Recently accessed items" "block"
+    And the page should meet accessibility standards
 
+  @accessibility
   Scenario: User has accessed some items
     Given I change window size to "large"
     When I am on the "Test forum name" "forum activity" page
     And I follow "Dashboard"
     Then I should see "Test forum name" in the "Recently accessed items" "block"
     And I should not see "Show more items" in the "Recently accessed items" "block"
+    And the page should meet accessibility standards
 
   Scenario: User has accessed more than 3 items
     Given the following "activities" exist: