Adding XCTestPlans using XCScheme

Hi! great project super helpful!

I'm working on some DX tooling and was wondering whether it is possible to add test plans to a scheme using the [Xcodeproj::XCScheme](https://www.rubydoc.info/gems/xcodeproj/Xcodeproj/XCScheme) class.

At a glance such class exposes a `doc` readonly attribute but that would be to get the final XML, there doesn't seem to be an object based API to add the testplan, thinking of something like `add_test_plan(test_plan) => Object`, is this the case? is this a planned feature?

Start with the Xcodeproj::XCScheme class and its readonly doc attribute to understand the existing scheme representation. Investigate how Xcode test plans are represented in the scheme XML and whether an object-based API is feasible. Done means the project has a decided approach for adding test plans through XCScheme.

Written by the indexing model from the issue text.