Test `test_timer::alarm_fires()` times out

[SteveLauC]

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I have seen this test failure several times after bumping our Linux CI runner:

failures:

---- sys::test_timer::alarm_fires stdout ----
thread 'sys::test_timer::alarm_fires' panicked at test/sys/test_timer.rs:85:13:
Timeout waiting for SIGALRM


failures:
    sys::test_timer::alarm_fires

For example, you can find one here.