core/scheduler: fix flaky TestSchedulerWait clock rewind

[KaloyanTanev]

TestSchedulerWait intermittently failed under -race (expected 1, actual 0). After waitBeaconSync advances the fake clock, the slot-ticker goroutine calls clock.After(slot.Time.Sub(clock.Now())) with a non-positive duration for the current slot. A real clockwork clock fires immediately on a non-positive duration, but the test's testClock.Sleep did now.Add(d) unconditionally, rewinding the clock and racing with the test's assertion. Guard testClock.Sleep to be a no-op on non-positive durations, matching real clock semantics.

category: test
ticket: none

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.48%. Comparing base (7691f96) to head (b99eb24).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4726      +/-   ##
==========================================
- Coverage   65.49%   65.48%   -0.01%     
==========================================
  Files         246      246              
  Lines       30123    30142      +19     
==========================================
+ Hits        19729    19739      +10     
- Misses      10393    10402       +9     
  Partials        1        1              

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