ipn: simplify TestLocalLogLines, defer a Shutdown of its LocalBackend

The test's LocalBackend was not shut down (Shutdown both releases
resources and waits for its various goroutines to end). This should
fix the test race we were seeing. It definitely fixes the file
descriptor leak that preventing -race -count=500 from passing before.

ipn/loglines_test.go

@@ -5,6 +5,7 @@
 package ipn
 
 import (
+	"reflect"
 	"testing"
 	"time"
 
@@ -48,6 +49,7 @@ func TestLocalLogLines(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer lb.Shutdown()
 
 	// custom adjustments for required non-nil fields
 	lb.prefs = NewPrefs()
@@ -55,30 +57,10 @@ func TestLocalLogLines(t *testing.T) {
 	// hacky manual override of the usual log-on-change behaviour of keylogf
 	lb.keyLogf = logListen.Logf
 
-	// testing infrastructure
-	type linesTest struct {
-		name string
-		want []string
-	}
-
-	tests := []linesTest{
-		{
-			name: "after prefs",
-			want: []string{
-				"peer keys: %s",
-				"v%v peers: %v",
-			},
-		},
-		{
-			name: "after peers",
-			want: []string{},
-		},
-	}
-
-	testLogs := func(want linesTest) func(t *testing.T) {
+	testWantRemain := func(wantRemain ...string) func(t *testing.T) {
 		return func(t *testing.T) {
-			if linesLeft := logListen.Check(); len(linesLeft) != len(want.want) {
-				t.Errorf("got %v, expected %v", linesLeft, want)
+			if remain := logListen.Check(); !reflect.DeepEqual(remain, wantRemain) {
+				t.Errorf("remain %q, want %q", remain, wantRemain)
 			}
 		}
 	}
@@ -89,7 +71,7 @@ func TestLocalLogLines(t *testing.T) {
 	prefs.Persist = persist
 	lb.SetPrefs(prefs)
 
-	t.Run(tests[0].name, testLogs(tests[0]))
+	t.Run("after_prefs", testWantRemain("peer keys: %s", "v%v peers: %v"))
 
 	// log peers, peer keys
 	status := &wgengine.Status{
@@ -103,5 +85,5 @@ func TestLocalLogLines(t *testing.T) {
 	}
 	lb.parseWgStatus(status)
 
-	t.Run(tests[1].name, testLogs(tests[1]))
+	t.Run("after_peers", testWantRemain())
 }