os: new test TestOpenFileCreateExclDanglingSymlink fails on Plan 9

[millerresearch]

CL 672396 added a symlink test for #73702 which always fails on Plan9:

--- FAIL: TestOpenFileCreateExclDanglingSymlink (0.03s)
--- FAIL: TestOpenFileCreateExclDanglingSymlink/InRoot (0.02s)
    os_test.go:2306: symlink does_not_exist link: not supported by plan 9
--- FAIL: TestOpenFileCreateExclDanglingSymlink/NoRoot (0.02s)
    os_test.go:2306: symlink does_not_exist link: not supported by plan 9
FAIL
FAIL	os	44.703s
ok  	os/exec	4.552s

Plan 9 doesn't have symbolic links, so this test should be skipped.

[dmitshur]

CC @golang/plan9, @neild.

[gopherbot]

Change https://go.dev/cl/674697 mentions this issue: os: skip TestOpenFileCreateExclDanglingSymlink when no symlinks

[millerresearch]

@gopherbot please consider this for backport to 1.24

CL 672396, which caused this issue by breaking the plan9 gotip builders, has been backported to 1.23 and 1.24. So the plan9 builders for those branches are now broken too. It would be helpful to undo the damage at least to the 1.24 branch by backporting the correction.

[gopherbot]

Backport issue(s) opened: #75359 (for 1.24).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/704336 mentions this issue: [release-branch.go1.24] os: skip TestOpenFileCreateExclDanglingSymlink when no symlinks