adds 'staging' task to spectests rakefile
adds 'staging' task to spectests rakefile

The new task entry point allows for terser invocation:

  bundle exec rake staging

rather than:

  bundle exec rake spec:{app,mon}-staging

mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream,mark-in/securedrop-prov-upstream