os: skip TestOpenFileCreateExclDanglingSymlink when no symlinks
Skip this test on plan9, and any other platform that doesn't
have symlinks.
Fixes #73729
Change-Id: I8052db24ed54c3361530bd4f54c96c9d10c4714c
Reviewed-on: https://go-review.googlesource.com/c/go/+/674697
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Commit-Queue: Alan Donovan <adonovan@google.com>
Reviewed-by: Richard Miller <millerresearch@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>