runtime: establish happens-before between goroutine and bubble exit
synctest.Run waits for all bubbled goroutines to exit before returning.
Establish a happens-before relationship between the bubbled goroutines
exiting and Run returning.
For #67434
Change-Id: Ibda7ec2075ae50838c0851e60dc5b3c6f3ca70fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/647755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>