test_stale_preconfig_reference_is_retaken_on_a_fresh_boot read the host's
uptime after boot_reference() had read it to decide, and expected the
decision its own reading implied. On a CI runner the uptime crossed
BOOT_MAX_AGE_S (600 s) between the two readings: boot_reference retook
the reference and the test expected the stale one (forgetest-ci on
0eb0a16). The test now holds uptime_s() at 60 s under and over the limit
and judges both answers: retaken with the fresh sysfs under it, the stale
reference kept and marked over it.
Proof: test_baseline passes on Windows and on Linux; with
boot_reference() ignoring the uptime (a planted fault) the test fails in
all three classes that run it.