--dont-add-python-opts may reduce the effectiveness of the tests: see #138805 (comment).

However, resolving this in the Android testbed required a fairly significant refactoring, so since this PR doesn't make the iOS situation any worse, it's probably best left to a separate PR.

I'm not sure I agree it's reducing effectiveness. It's preventing the use of a code branch that would otherwise be invoked as a consequence of using --fast-ci or --slow-ci; if that problem code branch is invoked, the test suite crashes. Using --fast-ci/--slow-ci improves testing consistency; the options that aren't being processed as a result of using this flag weren't being used previously, either.

I agree that the Python options that are enabled by that code branch should be factored into the iOS testbed - but as you have flagged, that would requires a much more substantial refactor, and it can be handled independently.

Sounds good. FYI, the reason why I made the Android testbed accept Python options on the command line, rather than hard-coding the options expected by the CPython test suite, is that some of those options might not be appropriate for third-party package tests run by cibuildwheel.

Sure - that makes sense. I've got some ideas for how this could be managed in the context of the iOS testbed - we can read options out of the Info.plist file to separate test suite options from interpreter options.

I found it simpler not to separate them at all, but to let PyConfig_SetBytesArgv handle both of them at once, just like in a normal Python executable.