This showed up as a flake, where we "got lucky" and the sendpay resolved before waitsendpay was called. Instead, make this race explicit, so we can test it.
```
# FIXME: #define PAY_UNPARSEABLE_ONION 202
PAY_UNPARSEABLE_ONION = 202
> assert err.value.error['code'] == PAY_UNPARSEABLE_ONION
E assert 204 == 202
tests/test_misc.py:2152: AssertionError
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>