forceCloseSockets is just designed as a kingston valve for TestMain
function and is not suitable to keep track of inflight sockets.

Fixes #15525.

Change-Id: Id967fe5b8da99bb08b699cc45e07bbc3dfc3ae3d
Reviewed-on: https://go-review.googlesource.com/23505Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>