Fix flaky test due to memoization

`ReleaseHighlight.file_paths` memoizes @file_paths
in a class instance variable. As a result, when this
class method is called across tests, the value persists
and subsequent tests would fail.

spec/models/release_highlight_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe ReleaseHighlight do
+RSpec.describe ReleaseHighlight, :clean_gitlab_redis_cache do
   let(:fixture_dir_glob) { Dir.glob(File.join('spec', 'fixtures', 'whats_new', '*.yml')).grep(/\d*\_(\d*\_\d*)\.yml$/) }
 
   before do

spec/requests/whats_new_controller_spec.rb

@@ -2,7 +2,11 @@
 
 require 'spec_helper'
 
-RSpec.describe WhatsNewController do
+RSpec.describe WhatsNewController, :clean_gitlab_redis_cache do
+  after do
+    ReleaseHighlight.instance_variable_set(:@file_paths, nil)
+  end
+
   describe 'whats_new_path' do
     let(:item) { double(:item) }
     let(:highlights) { double(:highlight, items: [item], map: [item].map, next_page: 2) }