sync: deflake TestWaitGroupMisuse3

Previous flakes:
https://build.golang.org/log/223365dedb6b6aa0cfdf5afd0a50fd433a16bade
https://build.golang.org/log/edbea4cd3f24e707ef2ae8378559bb0fcc453c22

Dmitry says in email about this:

> The stack trace points to it pretty clearly. Done can indeed unblock
> Wait first and then panic. I guess we need to recover after first
> Done as well.

And it looks like TestWaitGroupMisuse2 was already hardened against
this.  Do the same in TestWaitGroupMisuse3.

Change-Id: I317800c7e46f13c97873f0873c759a489dd5f47d
Reviewed-on: https://go-review.googlesource.com/19183


Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>