`test_uptime` is intermittent

[sylvestre]

Sometimes fails with (mac only?) Example: https://github.com/uutils/coreutils/runs/6857270607?check_suite_focus=true

---- test_uptime::test_uptime stdout ----
current_directory_resolved: 
run: /Users/runner/work/coreutils/coreutils/target/x86_64-apple-darwin/debug/coreutils uptime
thread 'test_uptime::test_uptime' panicked at 'Command was expected to succeed.
stdout = 
 stderr = uptime: could not retrieve system uptime
', tests/common/util.rs:174:9

[Joining7943]

test_uptime::test_uptime_since also fails sometimes on macos (from ci run https://github.com/uutils/coreutils/actions/runs/3713786104/jobs/6296860795)

---- test_uptime::test_uptime_since stdout ----
current_directory_resolved: 
run: /Users/runner/work/coreutils/coreutils/target/debug/coreutils uptime --since
---- test_uptime::test_uptime_since stderr ----
thread 'main' panicked at 'Command was expected to succeed.
stdout = 
 stderr = uptime: could not retrieve system uptime
', tests/common/util.rs:177:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace