testing/synctest: call out common issues with tests
While each point is explained their respective sections for Time,
Blocking, and Isolation, these appear to be the most common
issues worth calling out up front.
Fixes #75052
Change-Id: I916b559075ee4d1d23810a08459e037e21690193
Reviewed-on: https://go-review.googlesource.com/c/go/+/696736
Auto-Submit: Damien Neil <dneil@google.com> Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>