Improve init command to detect test targets as such

Currently, when I run something like this:

```bash
accio init --project-name "Demo" --target-name "Demo,DemoTests"
```

Accio will create a `Package.swift` manifest file with to `.target`s, but what I actually expect is that it automatically detects that the second target actually is of `.testTarget` type and automatically initializes it like that.