pytest: fix race in test_pay_direct.

I got a spurious failure because the final node gave a CLTV error and
so it decided to use a different channel.  It should probably handle
this corner case better, but meanwhile make the test robust.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_pay.py

@@ -1374,6 +1374,9 @@ def test_pay_direct(node_factory, bitcoind):
     # Let channels lock in.
     bitcoind.generate_block(5)
 
+    # Make l1 sees it, so it doesn't produce bad CLTVs.
+    sync_blockheight(bitcoind, [l1])
+
     # Make sure l0 knows the l2->l3 channel.
     # Without DEVELOPER, channel lockin can take 30 seconds to detect,
     # and gossip 2 minutes to propagate