Stabilize butex stop-after-running test

[wasphin]

What problem does this PR solve?

Problem Summary:

ButexTest.stop_after_running assumes that stopping and joining a task finishes
within 25 ms of a 10 ms sleep. The assertion is sensitive to scheduler delays
and has failed in CI exactly at the 25 ms boundary. The waiter already verifies
that bthread_stop() interrupts butex_wait() with EINTR, which is the
behavior under test.

What is changed and the side effects?

Changed:

Remove the wall-clock timing assertion and its timer while retaining the
EINTR, stop, join, and repeated-stop checks.

Side effects:

• Performance effects: None.

• Breaking backward compatibility: No.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.

[wasphin]

The failed clang-unittest-with-bazel job hit the unrelated flaky case SocketTest.socket_buffer_options_before_accept. Its fix is already covered by #3545. I have rerun the failed job and kept this PR scoped to ButexTest.stop_after_running.

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

This PR stabilizes the stop_after_running butex test by removing scheduler-sensitive timing validation.

Changes:

• Removes the timer and wall-clock assertion.
• Retains EINTR, stop, join, and repeated-stop checks.

File	Description
test/​bthread_butex_unittest.cpp	Stabilizes the stop-after-running test.

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[chenBright]

LGTM