It's not reliable:
```
# We should have deferred hook update at least once!
> l2.daemon.wait_for_log("UNUSUAL plugin-dep_b.py: Deferring registration of hook htlc_accepted until it's not in use.")
tests/test_plugin.py:2646:
...
if self.is_in_log(r):
print("({} was previously in logs!)".format(r))
> raise TimeoutError('Unable to find "{}" in logs.'.format(exs))
E TimeoutError: Unable to find "[re.compile("UNUSUAL plugin-dep_b.py: Deferring registration of hook htlc_accepted until it's not in use.")]" in logs.
```
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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