Add handling for Swift Testing Only Parallelization

[kelvinharron]

Related to this Tuist issue

Short description 📝

This PR introduces XcodeProj Parallelization handling for the new Swift Testing option. This will enable us to expose the option to Tuist TestingOptions allowing users to specify the option they need, as the default handling in Xcode 16 for Parallelization is going to benefit the few users who have adopted Swift Testing.

I also removed two dead links from the References section as I was going through the reading material.

Solution 📦

From changing the values manually in a real project, I gathered:

Parallelization off in Xcode ->"parallelizable" : false, Parallelization on in Xcode ->"parallelizable" : true, Parallelization Swift Testing Only in Xcode -> no value.

With guidance from @fortmarek , I updated the handling of the attributes to cover each individual case.

I've set it as draft as I need guidance on how to prove this handling outside of unit tests. Please let me know what I could be missing and how to verify the output. Thanks.

Implementation 👩‍💻👨‍💻

• [x] Introduce enums to cover the three unique cases
• [x] Add handling for each case from.
• [x] Update tests to handle each case.