all: close fake userspace engines when tests complete

We were leaking FDs.
In a few places, switch from defer to t.Cleanup.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>

ipn/ipnlocal/loglines_test.go

@@ -52,6 +52,7 @@ func TestLocalLogLines(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	t.Cleanup(e.Close)
 
 	lb, err := NewLocalBackend(logf, idA.String(), store, e)
 	if err != nil {