zip: skip symlink test on Plan 9

Fixes golang/go#36041

Change-Id: I7b65ca053e994c56d6a354133daff81bcadd7220
Reviewed-on: https://go-review.googlesource.com/c/mod/+/210397
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/zip/zip_test.go b/zip/zip_test.go
index fa32157..abdbefc 100644
--- a/zip/zip_test.go
+++ b/zip/zip_test.go
@@ -271,9 +271,10 @@
 			desc: "ignore_symlink",
 			setup: func(t *testing.T, tmpDir string) string {
 				if err := os.Symlink(tmpDir, filepath.Join(tmpDir, "link")); err != nil {
-					if runtime.GOOS == "windows" {
+					switch runtime.GOOS {
+					case "plan9", "windows":
 						t.Skipf("could not create symlink: %v", err)
-					} else {
+					default:
 						t.Fatal(err)
 					}
 				}