Ensure the correct Merge Request button is found

The project was not public and this caused a 404. So the tests were
giving deceiving results.

By searching for the Merge Request button in `#content-body` this
is overcome, and also other Merge Request buttons (e.g. in the
sidebar) are ignored.

spec/features/projects/merge_request_button_spec.rb

@@ -3,16 +3,18 @@ require 'spec_helper'
 feature 'Merge Request button', feature: true do
   shared_examples 'Merge Request button only shown when allowed' do
     let(:user) { create(:user) }
-    let(:project) { create(:project) }
-    let(:forked_project) { create(:project, forked_from_project: project) }
+    let(:project) { create(:project, :public) }
+    let(:forked_project) { create(:project, :public, forked_from_project: project) }
 
     context 'not logged in' do
       it 'does not show Create Merge Request button' do
         visit url
 
+        within("#content-body") do
           expect(page).not_to have_link(label)
         end
       end
+    end
 
     context 'logged in as developer' do
       before do
@@ -28,9 +30,11 @@ feature 'Merge Request button', feature: true do
 
         visit url
 
+        within("#content-body") do
           expect(page).to have_link(label, href: href)
         end
       end
+    end
 
     context 'logged in as non-member' do
       before do
@@ -40,8 +44,10 @@ feature 'Merge Request button', feature: true do
       it 'does not show Create Merge Request button' do
         visit url
 
+        within("#content-body") do
           expect(page).not_to have_link(label)
         end
+      end
 
       context 'on own fork of project' do
         let(:user) { forked_project.owner }
@@ -54,11 +60,13 @@ feature 'Merge Request button', feature: true do
 
           visit fork_url
 
+          within("#content-body") do
             expect(page).to have_link(label, href: href)
           end
         end
       end
     end
+  end
 
   context 'on branches page' do
     it_behaves_like 'Merge Request button only shown when allowed' do