gh-127146: Skip test_readinto_non_blocking on Emscripten

[hoodmane]

non_blocking doesn't really work on Emscripten.

• Issue: Emscripten: Get test suite passing #127146

[freakboy3742]

👍

[cmaloney]

@hoodmane, @freakboy3742 rather than a platform-specific check for this one test, could you make it so the set_blocking / get_blocking general platform feature flag which the test is also guarded by fails instead? Are a number of other tests in test_os that use that sort of check. Worried this platform specific check would end up needing to be copied to general libraries to support Emscripten. Python's io library has strong non-blocking support in its lower layers, and a number of projects / libraries rely on specific guarantees + file bugs and fixes.

[freakboy3742]

@cmaloney That's definitely an approach that we could take - it's an approach that iOS and Android use for a bunch of APIs in the "they exist, but you can't call them" category. However, in this case, it's not 100% clear if the issue here is a bug in Emscripten itself. If it is, then a test skip seems more appropriate. If it turns out there is a fundamental limitation that prevents Emscripten from avoiding the problem causing this test failure, then I'd agree guarding the entire feature would make sense.