ipn/ipnlocal: fix state_test data race seen in CI

Unfortunately I closed the tab and lost it in my sea of CI failures
I'm currently fighting.

Updates #cleanup

Change-Id: I4e3a652d57d52b75238f25d104fc1987add64191
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -652,7 +652,9 @@ func runTestStateMachine(t *testing.T, seamless bool) {
 	}
 
 	// undo the state hack above.
+	b.mu.Lock()
 	b.state = ipn.Starting
+	b.mu.Unlock()
 
 	// User wants to logout.
 	store.awaitWrite()