feature/linuxdnsfight: move inotify watching of /etc/resolv.conf out to a feature

tsnet apps in particular never use the Linux DNS OSManagers, so they don't need
DBus, etc. I started to pull that all out into separate features so tsnet doesn't
need to bring in DBus, but hit this first.

Here you can see that tsnet (and the k8s-operator) no longer pulls in inotify.

Updates #17206

Change-Id: I7af0f391f60c5e7dbeed7a080346f83262346591
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/direct_linux_test.go

@@ -1,61 +0,0 @@
-// Copyright (c) Tailscale Inc & AUTHORS
-// SPDX-License-Identifier: BSD-3-Clause
-
-package dns
-
-import (
-	"context"
-	"errors"
-	"fmt"
-	"os"
-	"sync/atomic"
-	"testing"
-	"time"
-
-	"golang.org/x/sync/errgroup"
-)
-
-func TestWatchFile(t *testing.T) {
-	dir := t.TempDir()
-	filepath := dir + "/test.txt"
-
-	ctx, cancel := context.WithCancel(context.Background())
-	defer cancel()
-
-	var callbackCalled atomic.Bool
-	callbackDone := make(chan bool)
-	callback := func() {
-		// We only send to the channel once to avoid blocking if the
-		// callback is called multiple times -- this happens occasionally
-		// if inotify sends multiple events before we cancel the context.
-		if !callbackCalled.Load() {
-			callbackDone <- true
-			callbackCalled.Store(true)
-		}
-	}
-
-	var eg errgroup.Group
-	eg.Go(func() error { return watchFile(ctx, dir, filepath, callback) })
-
-	// Keep writing until we get a callback.
-	func() {
-		for i := range 10000 {
-			if err := os.WriteFile(filepath, []byte(fmt.Sprintf("write%d", i)), 0644); err != nil {
-				t.Fatal(err)
-			}
-			select {
-			case <-callbackDone:
-				return
-			case <-time.After(10 * time.Millisecond):
-			}
-		}
-	}()
-
-	cancel()
-	if err := eg.Wait(); err != nil && !errors.Is(err, context.Canceled) {
-		t.Error(err)
-	}
-	if !callbackCalled.Load() {
-		t.Error("callback was not called")
-	}
-}