Include all generated XCTests into an Xcode test target

[bizz84]

Purpose: Keep AcceptanceMark Test target up to date as input files are modified.

By running amtool, all the required test files are written into an output folder.

Unless amtool keeps the test target up to date, the user is responsible for making sure that all the test files are included in the test target, which is error prone and a time drain.

We need a way of hooking into the client Xcode project to keep the test target up to date.