testing: allow parallel-subtest goroutines to exit when the subtest is complete

Fixes #45127
Updates #38768

Change-Id: I7f41901d5bcc07741ac9f5f2a24d2b07ef633cb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/303330


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>