Agree with @henrikbjorn, this was ensuring that if you call `setContent` on the source converter context that the source would have `setContent` called on it with the same value as its input.

This test would fail if you removed the `$sourceConverterContext->setContent('hello world');` line or, as @henrikbjorn pointed out, you set any other value than "hello world." It would also fail if you repeated the line again since it is expected to only be called once.