tests: raise dust limit on mpp test

Fails liquid-regtest otherwise; liquid tends to hit the dust limit
earlier than non-liquid tx, and MPP exacerbates this by divvying up
payments into dusty bits then attempting to shove them through the same
channel, hitting the dust max. The MPP then fails as not all the parts
were able to arrive at their destination.

tests/test_pay.py

@@ -3570,7 +3570,8 @@ def test_mpp_presplit(node_factory):
 
     l1, l2, l3 = node_factory.line_graph(
         3, fundamount=10**8, wait_for_announce=True,
-        opts={'wumbo': None}
+        opts={'wumbo': None,
+              'max-dust-htlc-exposure-msat': '500000sat'}
     )
 
     inv = l3.rpc.invoice(amt, 'lbl', 'desc')['bolt11']