Put loggers in before_action for easier overriding in EE

Make it easier to override the set of loggers, by putting them in a
separate method.

Also modify spec so it tests for links, instead of checking for
regular text.

app/controllers/admin/logs_controller.rb

 class Admin::LogsController < Admin::ApplicationController
+  before_action :loggers
+
   def show
-    @loggers = [
+  end
+
+  private
+
+  def loggers
+    @loggers ||= [
       Gitlab::AppLogger,
       Gitlab::GitLogger,
       Gitlab::EnvironmentLogger,

spec/features/admin/admin_browses_logs_spec.rb

@@ -8,8 +8,10 @@ describe 'Admin browses logs' do
   it 'shows available log files' do
     visit admin_logs_path
 
-    expect(page).to have_content 'test.log'
-    expect(page).to have_content 'githost.log'
-    expect(page).to have_content 'application.log'
+    expect(page).to have_link 'application.log'
+    expect(page).to have_link 'githost.log'
+    expect(page).to have_link 'test.log'
+    expect(page).to have_link 'sidekiq.log'
+    expect(page).to have_link 'repocheck.log'
   end
 end