Fixes the tests so that they can execute on their own.

Speed benchmarks:

```
$ rake
test:file[test/services/subscriber_to_feed_data_converter_test.rb]

6 tests, 1 assertions, 1 failures, 5 errors, 0 skips
```

```
$ rake
test:file[test/services/subscriber_to_feed_data_converter_test.rb]

Finished tests in 0.005167s, 1161.2156 tests/s, 2128.8953 assertions/s.

```

Fair enough.
Merge remote-tracking branch 'wilkie/bugfix/subscriber_to_feed_data_converter_test-speed'