[release-branch.go1.8] os: skip TestNetworkSymbolicLink if Server service is not...

[release-branch.go1.8] os: skip TestNetworkSymbolicLink if Server service is not started

Fixes #20179

Change-Id: I2b405c9a212a75aae628ad51885616d33c054191
Reviewed-on: https://go-review.googlesource.com/42190
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/68030
Run-TryBot: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>