tests: fix flaky test_sql blockheight race

The test mines ~106 blocks (channel close, replacement channel
funding, then generate_block(99)) immediately before making three
payments, without waiting for the nodes to process those blocks.
Under valgrind the paying node can lag far behind bitcoind when the
payments start, so it builds onions whose cltv expiries the caught-up
peers reject.  Two CI failures showed both shapes of the race:

In one run l1 was ~40 blocks behind (l2/l3 at block 210, l1 still
catching up), so l2 failed the forward: "Expiry cltv 181 too close to
current 210" (expiry_too_soon).  Since the error came from the
invoice's route-hint channel, xpay disabled it and the payment failed
hard: "All 1 channels to the destination are disabled."

In the other run l1 was only a few blocks behind, so l3 failed the
final hop: "Expiry cltv too soon 211 < 210 + 5".  xpay's
waitblockheight retry then succeeded, but the failed first attempt
left status 'failed' in the forwards row the test later asserts is
'settled'.

The test paid with pay until 7f6a333 ("pytest: use xpay, not pay in
misc tests."); pay took its start height from bitcoind's headercount
and tracked the node's chainlag, so a lagging node still built onions
valid at the true chain tip.  xpay uses its own notification-fed
blockheight plus one block of slack, which is why this only started
flaking recently.

Sync the nodes' blockheights before paying, as other tests do after
mining bursts.

Changelog-None

Author: ksedgwic

tests/test_plugin.py

@@ -4359,6 +4359,11 @@ def test_sql(node_factory, bitcoind):
     # Make sure we have a node_announcement for l1
     wait_for(lambda: l2.rpc.listnodes(l1.info['id'])['nodes'] != [])
 
+    # Under valgrind, the senders can still be digesting the blocks
+    # above and pick a stale blockheight for the payments below, which
+    # the caught-up peers reject as expiry_too_soon.
+    sync_blockheight(bitcoind, [l1, l2, l3])
+
     # This should create a forward through l2
     l1.rpc.xpay(l3.rpc.invoice(amount_msat=12300, label='inv1', description='description')['bolt11'])