net/http/httptest: make Server.CloseClientConnections wait for conns to close

net/http/httptest: make Server.CloseClientConnections wait for conns to close

httptest.Server was rewritten during Go 1.6, but
CloseClientConnections was accidentally made async in the rewrite and
not caught due to lack of tests.

Restore the Go 1.5 behavior and add tests.

Fixes #14290
Updates #14291

Change-Id: I14f01849066785053ccca2373931bc82d78c0a13
Reviewed-on: https://go-review.googlesource.com/19432
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>