gh-114099: Add test exclusions to support running the test suite on iOS (#114889)

Add test annotations required to run the test suite on iOS (PEP 730).

The majority of the change involve annotating tests that use subprocess,
but are skipped on Emscripten/WASI for other reasons, and including
iOS/tvOS/watchOS under the same umbrella as macOS/darwin checks.

`is_apple` and `is_apple_mobile` test helpers have been added to
identify *any* Apple platform, and "any Apple platform except macOS",
respectively.