os: mark the share created by TestNetworkSymbolicLink as temporary

os: mark the share created by TestNetworkSymbolicLink as temporary

Also use a unique share name for each run of the test.

This may help with #61467, but since I couldn't reproduce the failure
in the first place I don't know. It passes locally for me.

For #61467.

Change-Id: Ie51e3cf381063e02e4849af5c1a1ed7441ce21c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/512075
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>