Add feature test for emoji-only diff notes

This specs is related to bug described in #3734 (award-emojis).

features/project/commits/diff_comments.feature

@@ -13,6 +13,12 @@ Feature: Project Commits Diff Comments
     Given I leave a diff comment like "Typo, please fix"
     Then I should see a diff comment saying "Typo, please fix"
 
+  @javascript
+  Scenario: I can add a diff comment with a single emoji
+    Given I open a diff comment form
+    And I write a diff comment like ":smile:"
+    Then I should see a diff comment with an emoji image
+
   @javascript
   Scenario: I get a temporary form for the first comment on a diff line
     Given I open a diff comment form

features/steps/shared/diff_note.rb

@@ -87,6 +87,17 @@ module SharedDiffNote
     end
   end
 
+  step 'I write a diff comment like ":smile:"' do
+    page.within(diff_file_selector) do
+      click_diff_line(sample_commit.line_code)
+
+      page.within("form[rel$='#{sample_commit.line_code}']") do
+        fill_in 'note[note]', with: ':smile:'
+        click_button('Add Comment')
+      end
+    end
+  end
+
   step 'I submit the diff comment' do
     page.within(diff_file_selector) do
       click_button("Add Comment")
@@ -197,6 +208,12 @@ module SharedDiffNote
     end
   end
 
+  step 'I should see a diff comment with an emoji image' do
+    page.within("#{diff_file_selector} .note") do
+      expect(page).to have_xpath("//img[@alt=':smile:']")
+    end
+  end
+
   step 'I click side-by-side diff button' do
     find('#parallel-diff-btn').trigger('click')
   end