os: make tests pass even without Administrator account

This CL skips tests that call os.Symlink.

Updates #78340

Change-Id: I4d009112f32c45b4689660d0c01f638291c44f29
Reviewed-on: https://go-review.googlesource.com/c/go/+/782360
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>