Include all generated XCTests into an Xcode test target

## 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.

Start by tracing how `amtool` writes generated test files into the output folder and how the client Xcode project exposes its test target. Determine the supported way to update that target when input files change. Done means generated XCTests are automatically included without requiring users to maintain the target manually.

Written by the indexing model from the issue text.