pytest: fix race when we mine blocks after pay().

This seems to trigger now, especially on PostgresQL (maybe it's faster
to process blocks?).

e.g. test_closing_simple() hangs in close(), because the close is unilateral
because the HTLC timed out, so it's waiting for a block (other lines removed):

```
lightningd-1: 2022-01-12T00:33:46.258Z DEBUG   022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59-channeld-chan#1: peer_out WIRE_COMMITMENT_SIGNED
lightningd-1: 2022-01-12T00:33:46.278Z DEBUG   lightningd: close_command: timeout = 172800
2022-01-12T01:03:36.9757201Z lightningd-2: 2022-01-12T00:33:46.384Z DEBUG   lightningd: Adding block 104: 73ffa19d27d048613b2731e1682b4efff0dc226807d8cc99d724523c2ea58204
2022-01-12T01:03:36.9759053Z lightningd-2: 2022-01-12T00:33:46.396Z DEBUG   lightningd: Adding block 105: 44fd06ed053a0d0594abcfefcfa69089351fc89080826799fb4b278a68fe5c20
2022-01-12T01:03:36.9760865Z lightningd-2: 2022-01-12T00:33:46.406Z DEBUG   lightningd: Adding block 106: 0fee2dcbd1376249832642079131275e195bba4fb49cc9968df3a899010bba0f
2022-01-12T01:03:36.9762632Z lightningd-2: 2022-01-12T00:33:46.418Z DEBUG   lightningd: Adding block 107: 7f24f2d7d3e83fe3256298bd661e57cdf92b058440738fd4d7e1c8ef4a4ca073
2022-01-12T01:03:36.9773411Z lightningd-2: 2022-01-12T00:33:46.429Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#1: peer_in WIRE_REVOKE_AND_ACK
2022-01-12T01:03:36.9794707Z lightningd-2: 2022-01-12T00:33:46.437Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#1: Commits outstanding after recv revoke_and_ack
2022-01-12T01:03:36.9788197Z lightningd-2: 2022-01-12T00:33:46.433Z DEBUG   lightningd: Adding block 108: 283b371fb5d1ef42980ea10ab9f5965a179af8e91ddf31c8176e79820e1ec54d
2022-01-12T01:03:36.9799347Z lightningd-2: 2022-01-12T00:33:46.439Z DEBUG   0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-channeld-chan#1: HTLC 0[REMOTE] => RCVD_REMOVE_REVOCATION
2022-01-12T01:03:36.9808057Z lightningd-2: 2022-01-12T00:33:46.447Z UNUSUAL 0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518-chan#1: Peer permanent failure in CHANNELD_NORMAL: Fulfilled HTLC 0 RCVD_REMOVE_REVOCATION cltv 109 hit deadline
```

This is because `pay` returns from l1 when it has the preimage, not
when the HTLC is fully resolved.  Add a helper for this, and call it
at the end of the pay test helper.  We might need this elsewhere
though!

Signed-off-by: Rusty Russell <[email protected]>

contrib/pyln-testing/pyln/testing/utils.py

@@ -1021,6 +1021,9 @@ def pay(self, dst, amt, label=None):
         result = self.rpc.waitsendpay(rhash)
         assert(result.get('status') == 'complete')
 
+        # Make sure they're all settled, in case we quickly mine blocks!
+        dst.wait_for_htlcs()
+
     # This helper sends all money to a peer until even 1 msat can't get through.
     def drain(self, peer):
         total = 0