MDL-70829 mod_data: Mark up decorative images properly

* Decorative images should have an empty alt text and there's no need
to set a presentation role.
* Accessibility Behat tests added to cover the changes

mod/data/templates/view_noentries.mustache

@@ -55,14 +55,8 @@
     }
 }}
 <div class="text-xs-center text-center mt-4" data-region="empty-message">
-    <img
-            src="{{noitemsimgurl}}"
-            alt="{{#str}} norecords, mod_data {{/str}}"
-            role="presentation"
-            style="height: 70px; width: 70px;"
-    >
+    <img src="{{noitemsimgurl}}" alt="" style="height: 70px; width: 70px;">
     <h5 class="h5 mt-3 mb-0">{{#str}} norecords, mod_data {{/str}}</h5>
-
     <div class="mt-5 mb-0" id="action_bar">
         {{#importentriesbutton}}
             {{> core/single_button }}

mod/data/templates/zero_state.mustache

@@ -71,12 +71,7 @@
     }
 }}
 <div class="text-xs-center text-center mt-4" data-region="empty-message">
-    <img
-            src="{{noitemsimgurl}}"
-            alt="{{{ title }}}"
-            role="presentation"
-            style="height: 70px; width: 70px;"
-    >
+    <img src="{{noitemsimgurl}}" alt="" style="height: 70px; width: 70px;">
     <h5 class="h5 mt-3 mb-0">{{{ title }}}</h5>
     {{#intro}}
         <p class="mt-3 mb-0">{{{ intro }}}</p>

mod/data/tests/behat/view_entries.feature

@@ -183,3 +183,18 @@ Feature: Users can view and search database entries
       | param1 | param3 | autolink   |
       | 0      | 0      | should not |
       | 1      | 1      | should     |
+
+  @javascript @accessibility
+  Scenario: Check the accessibility of the database entries page (zero state)
+    When I am on the "Test database name" "data activity" page logged in as "teacher1"
+    Then I should see "No entries yet"
+    And the page should meet accessibility standards
+
+  @javascript @accessibility
+  Scenario: Check the accessibility of the database entries page
+    Given the following "mod_data > entries" exist:
+      | database | user     | Test field name | Test field 2 name      | Test field 3 name |
+      | data1    | student1 | Student entry 1 | Some student content 1 | http://moodle.com |
+      | data1    | teacher1 | Teacher entry 2 | Some teacher content 2 | http://moodle.com |
+    When I am on the "Test database name" "data activity" page logged in as teacher1
+    Then the page should meet accessibility standards

mod/data/tests/behat/zero_state.feature

@@ -65,3 +65,9 @@ Feature: Zero state page (no fields created)
     Then I click on "Use this preset" "button"
     And I should not see "Field mappings"
     And I should see "Image" in the "image" "table_row"
+
+  @javascript @accessibility
+  Scenario: Check the accessibility of the database activity zero state
+    When I am on the "Test database name" "data activity" page logged in as "teacher1"
+    Then I should see "Start building your activity"
+    And the page should meet accessibility standards