submarine_swaps: wait for broadcast in wait_for_htlcs_and_broadcast

[f321x]

Wait for broadcast of the funding transaction in SwapManager.wait_for_htlcs_and_broadcast before returning. Previously it might have returned the swap funding txid before the funding tx was broadcast.
This caused the regtest test_swapserver_success_forward to fail in the CI as it expected the funding tx to be broadcast once the CLI command returned.

See:

.***** test_swapserver_success_forward ******
initializing alice
funding alice
ec354953f96e0de7be4354ab156611da9e31bd85c2d25664bdb995950530c12e
initializing bob
funding bob
1df19a419b5ae5b961bae6cfaf4e7c8e772c6632d30782fab8de6992a8901e4a
mining 1 blocks
starting daemon (PID 7471)
/tmp/alice/regtest/wallets/default_wallet
true
starting daemon (PID 7502)
/tmp/bob/regtest/wallets/default_wallet
true

alice opens channel
mining 3 blocks
wait until alice sees channel open.
wait until alice sees channel open..
wait until alice sees channel open...
alice initiates forward-swap
{
  "lightning_amount": "0.01967661",
  "onchain_amount": "0.02",
  "txid": "b0f9ba48b670ee4a1f182655fe18f3f7d5c310923b7c85ad396b24e0a2f8e0cc"
}
utxo b0f9ba48b670ee4a1f182655fe18f3f7d5c310923b7c85ad396b24e0a2f8e0cc:0 does not exist
FDaemon stopped
Daemon stopped

======================================================================
FAIL: test_swapserver_success_forward (tests.regtest.TestLightningSwapserver.test_swapserver_success_forward)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/cirrus-ci-build/tests/regtest.py", line 105, in test_swapserver_success_forward
    self.run_shell(['swapserver_success_forward'])
  File "/tmp/cirrus-ci-build/tests/regtest.py", line 19, in run_shell
    assert process.returncode == 0
           ^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

Can be reproduced with:

diff --git a/electrum/submarine_swaps.py b/electrum/submarine_swaps.py
index f83d653f9..242aef070 100644
--- a/electrum/submarine_swaps.py
+++ b/electrum/submarine_swaps.py
@@ -1052,6 +1052,7 @@ class SwapManager(Logger):
     @log_exceptions
     async def broadcast_funding_tx(self, swap: SwapData, tx: Transaction) -> None:
         swap.funding_txid = tx.txid()
+        await asyncio.sleep(5)
         await self.network.broadcast_transaction(tx)

     async def reverse_swap(