gh-127146: Skip test_os.test_mode for Emscripten

[hoodmane]

This always fails in the build bot and always passes for me locally. Skip it for now in order to get the build bot running.

• Issue: Emscripten: Get test suite passing #127146

[freakboy3742]

Logged #135783 to track the underlying problem once this is merged.

[hoodmane]

Thanks!

[miss-islington-app]

Thanks @hoodmane for the PR, and @freakboy3742 for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

[bedevere-app]

GH-135784 is a backport of this pull request to the 3.14 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot aarch64 Android 3.14 (tier-3) has failed when building commit 2b5c904.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1794/builds/271) and take a look at the build logs.
4. Check if the failure is related to this commit (2b5c904) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1794/builds/271

Summary of the results of the build (if available):

Click to see traceback logs

Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/runners.py", line 118, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/base_events.py", line 687, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "/Users/android/buildarea/3.14.mhsmith-android-aarch64/build/Android/android.py", line 597, in run_testbed
    async with asyncio.TaskGroup() as tg:
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/taskgroups.py", line 134, in __aexit__
    raise propagate_cancellation_error
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/asyncio/taskgroups.py", line 110, in __aexit__
    await self._on_completed_fut
asyncio.exceptions.CancelledError

[freakboy3742]

The Android failure is an unrelated issue.