net: don't block forever in splice test cleanup on failure

The ppc64x builders are failing on the new splice test from CL 113997
but the actual failure is being obscured by a test deadlock.

Change-Id: I7747f88bcdba9776a3c0d2f5066cfec572706108
Reviewed-on: https://go-review.googlesource.com/133417


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>