gh-126167: Modify iOS Testbed to read arguments from Info.plist

[freakboy3742]

Modifies the iOS testbed so that:

1. Startup arguments are defined in the app's Info.plist, rather than hard-coded in source.
2. A new folder named app is added to PYTHONPATH to contain Python code.
3. A new folder named app_packages is added to PYTHONPATH to contain third-party modules (essentially a site-packages that isn't part of the Python framework)
4. The app and app_packages folders are processed by the script that converts libraries into frameworks
5. The unix working directory is set to the app folder.
6. NO_COLOR is set in the environment to prevent ANSI terminal codes from being emitted. This is required because the Xcode terminal output doesn't support color, and some test systems default to coloured output.

This doesn't alter how the testbed is started or used for CPython's test suite - the app and app_packages folders are empty by default, and aren't used by CPython.

However, a third party project can copy this project, add a Python.framework, add test code to app, install wheels into app_packages, rewrite the TestArgs key in iOSTestbed-Info.plist, and run the Xcode project to evaluate the test suite.

Fixes #126167.

• Issue: Genericize iOS testbed app #126167

[freakboy3742]

!buildbot iOS

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @freakboy3742 for commit f6ad3c0 🤖

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

The builders matched are:

• iOS ARM64 Simulator PR

[ned-deily]

The changes look reasonable. However, I wasn't able to fully test them yet due to #126925.

[freakboy3742]

@ned-deily Yeah - I've seen that, and I'm working on a fix that will both address that problem, fix the "don't get test results until the suite finishes" limitation, provide a useful utility for third parties to test their own code, and simplify the makefile target for CPython's iOS build.

That PR builds on this one, though; would you prefer me to merge that work into this PR, or would it be OK to merge this one as-is (on the basis CI verifies that it works as-is for Xcode 15.4)?

[ned-deily]

That PR builds on this one, though; would you prefer me to merge that work into this PR, or would it be OK to merge this one as-is (on the basis CI verifies that it works as-is for Xcode 15.4)?

I think either is fine, your call.

[miss-islington-app]

Thanks @freakboy3742 for the PR 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-126940 is a backport of this pull request to the 3.13 branch.

[freakboy3742]

Ok - I'll merge this one; I'm currently working on the other improvements. PyCon AU is later this week, which might slow my progress a little, so a PR might not be ready until late next week.