ipn/ipnlocal: close peerapi listeners on LocalBackend.Shutdown

For tests.

Now that we can always listen (whereas we used to fail prior to
a2c330c496), some goroutine leak
checks were failing in tests in another repo after that change.

Change-Id: Id95a4b71167eca61962a48616d79741b9991e0bc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/local.go

@@ -344,6 +344,7 @@ func (b *LocalBackend) onHealthChange(sys health.Subsystem, err error) {
 func (b *LocalBackend) Shutdown() {
 	b.mu.Lock()
 	cc := b.cc
+	b.closePeerAPIListenersLocked()
 	b.mu.Unlock()
 
 	b.unregisterLinkMon()