_This change should have no user impact._
We add some stuff to `String` in `SignalServiceKit`.
Before, we tested this in the `SignalTests` target. Now, it's in the
`SignalServiceKitTests` target.
All I did was move some lines. I didn't change the tests at all.
I think this is a good change on its own but it may be useful in an
upcoming change.
74d8d25a04