flaky CI macos: test_uptime_with_fifo fails due to changed error message(?)

[BenWiederhake]

The test test_uptime::test_uptime_with_fifo fails in steps Build (macos-13, x86_64-apple-darwin, feat_os_macos) and Build (macos-latest, aarch64-apple-darwin, feat_os_macos).

This is a real problem: It affects at least #6567 (https://github.com/uutils/coreutils/actions/runs/9935343852/job/27441455169), #6564 (https://github.com/uutils/coreutils/actions/runs/9935800829/job/27442853002?pr=6564 and https://github.com/uutils/coreutils/actions/runs/9935800829/job/27442853002?pr=6564), and probably many more PRs.

Typical stderr:

---- test_uptime::test_uptime_with_fifo stdout ----
mkfifo: /var/folders/1q/w28k7vpd4ln7kf4l0gfybp2h0000gn/T/.tmpLV3BOv/fifo1
write(default): /var/folders/1q/w28k7vpd4ln7kf4l0gfybp2h0000gn/T/.tmpLV3BOv/a
run: /Users/runner/work/coreutils/coreutils/target/x86_64-apple-darwin/debug/coreutils cp /var/folders/1q/w28k7vpd4ln7kf4l0gfybp2h0000gn/T/.tmpLV3BOv/a /var/folders/1q/w28k7vpd4ln7kf4l0gfybp2h0000gn/T/.tmpLV3BOv/fifo1
run: /Users/runner/work/coreutils/coreutils/target/x86_64-apple-darwin/debug/coreutils uptime fifo1
thread 'test_uptime::test_uptime_with_fifo' panicked at ''uptime: couldn't get boot time
' does not contain 'uptime: couldn't get boot time: Illegal seek'', tests/by-util/test_uptime.rs:77:10

This particular error message reads like the error message simply got slightly more general, and the test should be updated to reflect that.

This is yet another time that a test is flaky on MacOS, and makes CI red for unrelated PRs. See also #6275 #6333 #6419.