Revert "testing: simplify concurrency and cleanup logic"

Revert "testing: simplify concurrency and cleanup logic"

reverts CL 506755

Reason for revert: This leads to deadlocks of tests in Google internal testing

For #64402

Change-Id: I78329fc9dcc2377e7e880b264ac1d18d577ef99a
Reviewed-on: https://go-review.googlesource.com/c/go/+/544895
Auto-Submit: Bryan Mills <bcmills@google.com>
Auto-Submit: Lasse Folger <lassefolger@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>