ipn/ipnlocal: move where auto exit node selection happens

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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>

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