Fix flakey test

This failed on travis, but I couldn't reproduce the failure locally (see
below).

This change should allow the test to run reliably. The cause of the
intermittent failure remains a mystery.

https://travis-ci.org/alphagov/whitehall/builds/5982193

```
Finished tests in 291.241102s, 10.7471 tests/s, 16.7284 assertions/s.
  1) Error:
test_validation_errors_when_reordering_features_are_propogated(FeatureListTest):
Mocha::StubbingError: stubbing non-existent method:
[#<Feature:0xdeb7230>].find_by_id!
    /home/travis/.rvm/gems/ruby-1.9.2-p320/gems/activerecord-3.1.12/lib/active_record/associations/collection_proxy.rb:97:in `method_missing'
    /home/travis/build/alphagov/whitehall/test/unit/feature_list_test.rb:51:in `block in <class:FeatureListTest>'
    /home/travis/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.1.12/lib/active_support/testing/setup_and_teardown.rb:36:in `block in run'
    /home/travis/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.1.12/lib/active_support/callbacks.rb:417:in `_run_setup_callbacks'
    /home/travis/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.1.12/lib/active_support/callbacks.rb:81:in `run_callbacks'
    /home/travis/.rvm/gems/ruby-1.9.2-p320/gems/activesupport-3.1.12/lib/active_support/testing/setup_and_teardown.rb:35:in `run'
```