net: use testenv.SkipIfOptimizationOff in TestAllocs

net: use testenv.SkipIfOptimizationOff in TestAllocs

Use the helper added by CL 422038 instead of manually detecting the
noopt builder.

Change-Id: I353f1a9d5f0184d73869520eb1ae8d0bdbbe2006
Reviewed-on: https://go-review.googlesource.com/c/go/+/423855
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>