cmd/link: skip TestDWARF on Plan 9

TestDWARF has been added in CL 38855. This test is
failing on Plan 9 because executables don't have
a DWARF symbol table.

Fixes #19793.

Change-Id: I7fc547a7c877b58cc4ff6b4eb5b14852e8b4668b
Reviewed-on: https://go-review.googlesource.com/38931
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/dwarf_test.go

@@ -14,6 +14,7 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -21,6 +22,10 @@ import (
 func TestDWARF(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
 
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	out, err := exec.Command(testenv.GoToolPath(t), "list", "-f", "{{.Stale}}", "cmd/link").CombinedOutput()
 	if err != nil {
 		t.Fatalf("go list: %v\n%s", err, out)