Add additional target to Xcode project

[cptpatrick]

Hi, we use premake for quite a while for our cross-platform development (Windows/Android/iOS). Just recently we came to the point where we would add a XCTest target to our iOS projects. I found a PR#1218 which already added support for XCTest but now my question is: How would I apply that in my lua file? When adding a unittest bundle in XCode directly, it is added as a new target to an existing project. I see no code that would allow adding more than one target to a project with the current premake implementation and I didn't find any references on the internet where someone would have already done this... am I just missing something or is this just not possible at the moment?