Fix what `bolt2_open_channel_sending_node_checks_part1` tests

Fix what `bolt2_open_channel_sending_node_checks_part1` tests

There are currently two issues with
`bolt2_open_channel_sending_node_checks_part1` which counteract
each other and hide that the test isn't testing what it should be.

First of all, the final `create_channel` call actually fails
because we try to open a channel with ourselves, instead of
panicing as the test is supposed to check for.

However, when we fix the create_channel call to panic, when we
drop `nodes[1]` after `create_channel` panics, we fail the
no-pending-messages test as it as an expeted `accept_channel` in
its outbound buffer. This causes a double-panic.

Previously, these two offset each other - instead of panicing in
`create_channel` we'd panic in the Node drop checks.

This fixes both by fetching the `accept_channel` before we go into
the panic'ing `create_channel` call (who's arguments were
		corrected).