[release-branch.go1.8] 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.

(This fixes Plan 9 failures on the release branch introduced by
cherry-pick CL 39605.)

Change-Id: I7fc547a7c877b58cc4ff6b4eb5b14852e8b4668b
Reviewed-on: https://go-review.googlesource.com/39611
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/link/dwarf_test.go

@@ -26,6 +26,10 @@ 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)