test: replace time.Sleep with deterministic synchronization in unit tests

Replace three time.Sleep usages in tests with channel-based and
happens-before reasoning:

- internal/ior_mode_test.go: waitForStreamRows no longer polls with
  time.Sleep(1ms); starter() only returns after the trace goroutine
  closes the started channel, which happens after all printCb pushes,
  forming a happens-before edge that guarantees the rows are visible.

- internal/probemanager/manager_test.go: the intermediate attach-count
  assertions (enter==1, exit==0) are now checked immediately after
  <-enterBlocked, which is itself a happens-before edge, rather than
  after a 50ms sleep. The concurrent goroutine 2 is started afterwards;
  the final count assertions after both goroutines complete verify the
  serialization invariant.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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