pytest: don't run test_hook_in_use under VALGRIND on CI.

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>

Author: rustyrussell

tests/test_plugin.py

@@ -2614,6 +2614,7 @@ def test_htlc_accepted_hook_failonion(node_factory):
         l1.rpc.pay(inv)
 
 
+@pytest.mark.slow_test  # VALGRIND running generally too slow to trigger race we need.
 def test_hook_in_use(node_factory):
     """If a hook is in use when we add a plugin to it, we have to defer"""
     dep_a = os.path.join(os.path.dirname(__file__), 'plugins/dep_a.py')