net/dns: fix TestDNSTrampleRecovery failure under flakestress

The test had two problems:

1. runFileWatcher passed hardcoded "/etc/" to the inotify watcher,
   but the test filesystem uses a temp directory prefix. The watcher
   was watching the real /etc/, never seeing the test's file writes.

2. The test's watchFile used gonotify.NewDirWatcher which creates
   goroutines that block on real inotify syscalls. These don't work
   inside synctest's fake-time bubble. The test only passed standalone
   by accident: gonotify walks /etc/ on startup producing fake events
   that happened to trigger trample detection at the right time.

Fix the path issue by adding ActualPath to the wholeFileFS interface,
which translates logical paths (like "/etc/resolv.conf") to real
filesystem paths (respecting any test prefix). Use it in
runFileWatcher so the inotify watch targets the correct directory.

Replace gonotify in the test with a one-shot timer that synctest can
advance through fake time, reliably triggering the trample check.

Fixes #19400

Change-Id: Idb252881ec24d0ab3b3c1d154dbdaf532db837d4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/wsl_windows.go

@@ -148,6 +148,8 @@ type wslFS struct {
 	distro string
 }
 
+func (fs wslFS) ActualPath(name string) string { return name }
+
 func (fs wslFS) Stat(name string) (isRegular bool, err error) {
 	err = wslRun(fs.cmd("test", "-f", name))
 	if ee, _ := err.(*exec.ExitError); ee != nil {