gh-140189: Add CI job to test iOS builds.

[freakboy3742]

Adds a CI task to build and test an iOS XCframework.

The shape of this is almost identical to the Android task; the only notable difference is that it explicitly selects an Xcode version. Explicitly selecting an Xcode version is recommended by GitHub for all macOS activity, but changes rolled out to GitHub Actions during August made this a necessity, as the default Xcode configurations will not reliably start an iOS simulator (see actions/runner-images/issues/12541 and actions/runner-images/issues/12751).

• Issue: iOS CI and binary releases #140189

[freakboy3742]

Hrm... this works, but I get the impression something isn't quite working right with logging, because the group collapsing ins't working the same as it is on Android.

@mhsmith Any ideas? It looks like the output of the subprocesses is being piped entirely differently...

[hugovk]

This job takes about 34 minutes, making it the new bottleneck. The other slowest are free-threaded Windows on x64 (26m) and arm64 (24m):

Is there anything we can do to speed it up a bit?

It's running tests sequentially in a single process, which takes 17 minutes. Can we parallelise? Or cache any of the build process?

[StanFromIreland]

Is it possible to build without installing, maybe that would save some time? From the logs it looks like everything is installed:

/usr/bin/install -c -m 644 ../../Lib/__future__.py Apple/iOS/Frameworks/arm64-iphoneos/lib/python3.15

Also looking at the logs, I am not familiar with the iOS build system, but it looks as if configure is ran four or five times, is that intentional?