cmd/compile: skip TestScopeRanges on Plan 9

TestScopeRanges has been added in CL 40095. This
test is failing on Plan 9 because executables don't
have a DWARF symbol table.

Fixes #20418.

Change-Id: I6dd3baa636998134ccd042203c8b5c3199a4d6e1
Reviewed-on: https://go-review.googlesource.com/43670
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/scope_test.go

@@ -12,6 +12,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"sort"
 	"strconv"
 	"strings"
@@ -182,6 +183,11 @@ const detailOutput = false
 // corresponds to what we expect it to be.
 func TestScopeRanges(t *testing.T) {
 	testenv.MustHaveGoBuild(t)
+
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping on plan9; no DWARF symbol table in executables")
+	}
+
 	dir, err := ioutil.TempDir("", "TestScopeRanges")
 	if err != nil {
 		t.Fatalf("could not create directory: %v", err)