internal/testenv, syscall: move isNotSupported to internal/testenv

internal/testenv, syscall: move isNotSupported to internal/testenv

This allows to use this helper function in packages other than syscall,
namely package net.

For #58114

Change-Id: I72c59ab013e9195801ff1315019ae1aef4396287
Reviewed-on: https://go-review.googlesource.com/c/go/+/476216
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>