[3.14] gh-137973: Add a non-parallel test plan to the iOS testbed project (GH-138018)

[miss-islington]

Modifies the iOS testbed project to add a test plan. This simplifies the iOS
test runner, as we can now use the built-in log streaming to see test results.
It also allows for some other affordances, like providing a default LLDB config,
and using a standardized mechanism for specifying test arguments.
(cherry picked from commit 2ba2287)

Co-authored-by: Russell Keith-Magee russell@keith-magee.com

• Issue: iOS testbed runner incompatible with GitHub Actions macos-15 runner. #137973

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📚 Documentation preview 📚: https://cpython-previews--138038.org.readthedocs.build/

[freakboy3742]

!buildbot iOS

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @freakboy3742 for commit 6264320 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F138038%2Fmerge

The command will test the builders whose names match following regular expression: iOS

The builders matched are:

• iOS ARM64 Simulator PR

[freakboy3742]

@hugovk Assigning to you for consideration for inclusion in the next 3.14 RC.

This set of changes is required because the 20250811 changes to the macos-15 image for GitHub Actions has proven to be incompatible with the CPython iOS testbed. cibuildwheel uses this testbed to run package tests; at present, it isn't possible to run test iOS wheels on the macos-15 image.

[freakboy3742]

I've verified that this testbed works on GitHub Actions. The image still has some problems related to inconsistent startup times of the simulator, but the problems related to the Xcode project (i.e., the things under our control) appear to be resolved.