Fix warning about `toString` being unused when building tests.
Fix warning about `toString` being unused when building tests.

The overload of `toString` we were providing wasn't being picked up by
Catch, and so was never being called. Switch to their suggested
alternative approach of providing a specialization of
`StringMaker`. This specialization is correctly picked up by Catch.

realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-object-store,realm/realm-object-store,realm/realm-object-store,realm/realm-core,realm/realm-core,realm/realm-core,realm/realm-core