timer: rebase the next elapse timestamp only if timer didn't already run

timer: rebase the next elapse timestamp only if timer didn't already run

The test added in f4c3c107d9be4e922a080fc292ed3889c4e0f4a5 uncovered a
corner case while recalculating the next elapse timestamp of a timer unit
that uses RandomizedDelaySec= during deserialization.

If the scheduled time (without RandomizedDelaySec=) already elapsed,
systemd "rebases" the next elapse timestamp to the time when systemd
first started, to make the RandomizedDelaySec= feature work even at
boot. However, since it was done unconditionally, it always overrode the
next elapse timestamp, which could then cause the final next elapse
timestamp to fall out of the expected window.

With a couple of additional debug logs one of the test fail looks like
this:

[  132.129815] TEST-53-TIMER.sh[384]: + : 'Next elapse timestamp after daemon-reload, try #328'
[  132.129815] TEST-53-TIMER.sh[384]: + systemctl daemon-reload
[  132.136352] systemd[1]: Reload requested from client PID 16399 ('systemctl') (unit TEST-53-TIMER.service)...
[  132.136636] systemd[1]: Reloading...
[  132.446160] systemd[1]: Rebasing next elapse timestamp
[  132.446168] systemd[1]: v->next_elapse: Tue 2025-10-14 00:10:00 CEST
[  132.446170] systemd[1]: rebased: Tue 2025-10-14 00:10:56 CEST
[  132.446172] systemd[1]: v->next_elapse after rebase: Tue 2025-10-14 00:10:56 CEST
[  132.447361] systemd[1]: Reloading finished in 310 ms.
[  132.484041] TEST-53-TIMER.sh[384]: + check_elapse_timestamp
[  132.484041] TEST-53-TIMER.sh[384]: + systemctl status timer-RandomizedDelaySec-16377.timer
[  132.533657] TEST-53-TIMER.sh[16440]: ● timer-RandomizedDelaySec-16377.timer
[  132.533657] TEST-53-TIMER.sh[16440]:      Loaded: loaded (/run/systemd/system/timer-RandomizedDelaySec-16377.timer; static)
[  132.533657] TEST-53-TIMER.sh[16440]:      Active: active (waiting) since Mon 2025-10-13 23:00:00 CEST; 1h 13min ago
[  132.533657] TEST-53-TIMER.sh[16440]:  Invocation: 5555d4f060114a5493ff228013830d17
[  132.533657] TEST-53-TIMER.sh[16440]:     Trigger: Tue 2025-10-14 22:10:04 CEST; 21h left
[  132.533657] TEST-53-TIMER.sh[16440]:    Triggers: ● timer-RandomizedDelaySec-16377.service
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:07 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Changed dead -> waiting
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:07 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Adding 15h 35min 1.230173s random time.
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:07 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Realtime timer elapses at Tue 2025-10-14 15:45:58 CEST.
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:07 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Changed dead -> waiting
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:08 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Adding 16h 29min 44.084409s random time.
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:08 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Realtime timer elapses at Tue 2025-10-14 16:40:41 CEST.
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:08 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Changed dead -> waiting
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:08 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Adding 21h 59min 7.955828s random time.
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:08 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Realtime timer elapses at Tue 2025-10-14 22:10:04 CEST.
[  132.533657] TEST-53-TIMER.sh[16440]: Oct 14 00:13:08 H systemd[1]: timer-RandomizedDelaySec-16377.timer: Changed dead -> waiting
[  132.535386] TEST-53-TIMER.sh[384]: + systemctl show -p InactiveExitTimestamp timer-RandomizedDelaySec-16377.timer
[  132.537727] TEST-53-TIMER.sh[16442]: InactiveExitTimestamp=Mon 2025-10-13 23:00:00 CEST
[  132.540317] TEST-53-TIMER.sh[16444]: ++ systemctl show -P NextElapseUSecRealtime timer-RandomizedDelaySec-16377.timer
[  132.547745] TEST-53-TIMER.sh[384]: + NEXT_ELAPSE_REALTIME='Tue 2025-10-14 22:10:04 CEST'
[  132.548020] TEST-53-TIMER.sh[16445]: ++ date '--date=Tue 2025-10-14 22:10:04 CEST' +%s
[  132.550218] TEST-53-TIMER.sh[384]: + NEXT_ELAPSE_REALTIME_S=1760472604
[  132.550218] TEST-53-TIMER.sh[384]: + : 'Next elapse timestamp should be Tue 2025-10-14 00:10:00 CEST <= Tue 2025-10-14 22:10:04 CEST <= Tue 2025-10-14 22:10:00 CEST'
[  132.550218] TEST-53-TIMER.sh[384]: + assert_ge 1760472604 1760393400
[  132.550555] TEST-53-TIMER.sh[16446]: + set +ex
[  132.550702] TEST-53-TIMER.sh[384]: + assert_le 1760472604 1760472600
[  132.550832] TEST-53-TIMER.sh[16447]: + set +ex
[  132.551091] TEST-53-TIMER.sh[16447]: FAIL: '1760472604' > '1760472600'

Here the original next elapse timestamp was Tue 2025-10-14 00:10:00 CEST
as expected, but it was overridden by the rebased timestamp:
Tue 2025-10-14 00:10:56 CEST. And when a new randomized delay was added
to it (21h 59min 7.955828s) the final next elapse timestamp fell out of
the expected window, i.e. Tue 2025-10-14 00:10:00 (scheduled time) <
Tue 2025-10-14 22:10:04 CEST (rebased elapse timestamp + randomized
delay) < Tue 2025-10-14 22:10:00 CEST (scheduled time + maximum from
RandomizedDelaySec=, i.e. 22h).

By limiting the timestamp rebase only the case where the unit hasn't
already run should prevent this from happening during daemon-reload.