In the process, because I needed it for testing, make all
LocalBackend-managed goroutines be accounted for. And then in tests,
verify they're no longer running during LocalBackend.Shutdown.
Updates tailscale/corp#19681
Change-Id: Iad873d4df7d30103a4a7863dfacf9e078c77e6a3
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>