net, runtime: drop macOS 10.12 skip conditions in tests

net, runtime: drop macOS 10.12 skip conditions in tests

Go 1.17 requires macOS 10.13 or later. Thus, drop the special cases for
the darwin-amd64-10_12 builder added in CL 202618.

Updates #22019
Updates #23011
Updates #32919

Change-Id: Idef11c213dfb25fd002b7cda6d425cf2e26a2e06
Reviewed-on: https://go-review.googlesource.com/c/go/+/318329
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>