test: fix flaky ScheduledExecutorsTest.testscheduleWithFixedDelay

[ykisana]

Fixes #20503.

Description

ScheduledExecutorsTest.testscheduleWithFixedDelay asserted that the first task starts strictly more than 100ms after scheduling (firstTaskStart > 100). The test measured with System.currentTimeMillis(), but ScheduledThreadPoolExecutor schedules on System.nanoTime() and can fire exactly on time. A real gap of 100.0–100.99ms truncates to 100 ms, so the strict bound failed even though the executor behaved correctly (CI: was: 100 on all 4 surefire attempts).

Fixed the flaky lower bound

• Record task start times with System.nanoTime(), relative to the start of the test, so the test uses the same monotonic clock as the executor and isn't affected by wall-clock adjustments.
• Make the lower bound inclusive (firstTaskStart >= 100). schedule() is called after the start time is recorded, so the measured gap can never be less than the initial delay.
• Replace the comment that claimed the delay is always greater than 100ms "due to overhead", which was the incorrect assumption behind the flake.

The checks on the delay between later tasks are unchanged. Every start time is measured from the same origin, so the gaps between them are the same as before.

---

Key changed/added classes in this PR

• ScheduledExecutorsTest

---

This PR has:

• been self-reviewed.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.