[release-branch.go1.11] runtime: skip TestLockOSThreadAvoidsStatePropagation if one can't unshare

This change splits a testprog out of TestLockOSThreadExit and makes it
its own test. Then, this change makes the testprog exit prematurely with
a special message if unshare fails with EPERM because not all of the
builders allow the user to call the unshare syscall.

Also, do some minor cleanup on the TestLockOSThread* tests.

Fixes #29366.

Change-Id: Id8a9f6c4b16e26af92ed2916b90b0249ba226dbe
Reviewed-on: https://go-review.googlesource.com/c/155437


Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit 429bae71)
Reviewed-on: https://go-review.googlesource.com/c/go/+/167707


Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>