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/manager_linux_test.go

@@ -316,6 +316,7 @@ func (m memFS) Stat(name string) (isRegular bool, err error) {
 func (m memFS) Chmod(name string, mode os.FileMode) error { panic("TODO") }
 func (m memFS) Rename(oldName, newName string) error      { panic("TODO") }
 func (m memFS) Remove(name string) error                  { panic("TODO") }
+func (m memFS) ActualPath(name string) string             { return name }
 func (m memFS) ReadFile(name string) ([]byte, error) {
 	v, ok := m[name]
 	if !ok {