tests: make test_lockup_drain a lot less flaky

The feerate at which a 'forced feerate' causes capacity to be exceeded is
incredibly variable

FIXME: investigate why, but for now use a feerate that's high enough to
always trigger the capacity exceeded check

tests/test_pay.py

@@ -2410,27 +2410,28 @@ def test_channel_spendable_receivable_capped(node_factory, bitcoind):
     assert l2.rpc.listpeers()['peers'][0]['channels'][0]['receivable_msat'] == Millisatoshi(0xFFFFFFFF)
 
 
-@unittest.skipIf(True, "Test is extremely flaky")
 @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})
+    l1, l2 = node_factory.line_graph(2, opts={'may_reconnect': True,
+                                              'feerates': (15000, 15000, 15000, 15000)})
 
     # l1 sends all the money to l2 until even 1 msat can't get through.
     total = l1.drain(l2)
 
-    # Even if feerate now increases 2x (30000), l2 should be able to send
+    # Even if feerate now increases, l2 should be able to send
     # non-dust HTLC to l1.
     l1.force_feerates(30000)
+    assert total != 0
     l2.pay(l1, total // 2)
 
     # reset fees and send all back again
     l1.force_feerates(15000)
     l1.drain(l2)
 
-    # But if feerate increase just a little more, l2 should not be able to send
-    # non-fust HTLC to l1
-    l1.force_feerates(30002)  # TODO: Why does 30001 fail? off by one in C code?
+    # But if feerate increases, l2 should not be able to send
+    # non-dust HTLC to l1
+    l1.force_feerates(35000)  # TODO: Why is this flaky af
     wait_for(lambda: l1.rpc.listpeers()['peers'][0]['connected'])
     with pytest.raises(RpcError, match=r".*Capacity exceeded.*"):
         l2.pay(l1, total // 2)