cmd/go/internal/fsys: skip symlink test on Plan 9

Fixes #41950
Fixes #41954

Change-Id: I95d97f076fa928f3638309b78748d7ccc7277b14
Reviewed-on: https://go-review.googlesource.com/c/go/+/261897
Trust: Jay Conrod <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: fhs

src/cmd/go/internal/fsys/fsys_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -654,6 +655,8 @@ func TestWalk_Error(t *testing.T) {
 }
 
 func TestWalk_Symlink(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
 	initOverlay(t, `{
 	"Replace": {"overlay_symlink": "symlink"}
 }