CI: flaky Rust integration tests on test_timeout and test_tail #15684

Two Rust integration tests fail intermittently in CI and are unrelated to the PRs they block. Both pass on a clean local re-run.

Affected tests

• test_timeout::test_zero_kill_after_does_not_escalate_on_sigint — fails on macOS build jobs (aarch64/x86_64). Signal and timing sensitive.
• test_tail::test_tail_obsolete_bare_c_needs_posix_1992 — fails on the ubuntu Tests job with failed to write to stdin of child: Broken pipe (os error 32) (tests/uutests/src/lib/util.rs). The test writes to the child's stdin and races the child exiting, so the write hits a closed pipe.

Evidence — observed on PR #13684 (a uucore fs change that touches neither test); both pass locally when run in isolation.

Scope — distinct from #6732 (GNU tests/timeout/timeout) and broader than #12353, which lists a different set of tests.

Proposed direction

• timeout test: make signal-escalation timing deterministic rather than wall-clock dependent, or widen the tolerance.
• tail test: tolerate a broken pipe when the child has already exited instead of unwrapping the stdin write in the test harness.