pytest: Skip test_lockup_drain if VALGRIND and not DEVELOPER

The test was failing very reliably, by not raising the expected exception. My
best guess is that it needs more synchronization to cause the issue.

tests/test_pay.py

@@ -6,7 +6,7 @@ from pyln.client import RpcError, Millisatoshi
 from pyln.proto.onion import TlvPayload
 from utils import (
     DEVELOPER, wait_for, only_one, sync_blockheight, TIMEOUT,
-    EXPERIMENTAL_FEATURES, env
+    EXPERIMENTAL_FEATURES, env, VALGRIND
 )
 import copy
 import os
@@ -2320,6 +2320,7 @@ def test_channel_spendable_receivable_capped(node_factory, bitcoind):
     assert l2.rpc.listpeers()['peers'][0]['channels'][0]['receivable_msat'] == Millisatoshi(0xFFFFFFFF)
 
 
+@unittest.skipIf(not DEVELOPER and VALGRIND, "Doesn't raise exception, needs better sync")
 def test_lockup_drain(node_factory, bitcoind):
     """Try to get channel into a state where opener can't afford fees on additional HTLC, so peer can't add HTLC"""
     l1, l2 = node_factory.line_graph(2, opts={'may_reconnect': True})