Add basic feature spec for Balsamiq viewer

spec/features/projects/blobs/balsamiq_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Balsamiq file blob', :js do
+  let(:project) { create(:project, :public, :repository) }
+
+  before do
+    visit project_blob_path(project, 'add-balsamiq-file/files/images/balsamiq.bmpr')
+
+    wait_for_requests
+  end
+
+  it 'displays Balsamiq file content' do
+    expect(page).to have_content("Mobile examples")
+  end
+end