@neild requested issue #74837 to be considered for backport to the next 1.25 minor release.

@gopherbot please backport to go1.25.

We don't normally backport non-bugfix behavioral changes, but testing/synctest is a brand-new API and I think we're better off minimizing the number of releases with the old behavior. The change is limited in scope to tests using synctest, and only affects buggy tests, so the risk should be very low.