pytest: generalize tests.

Anchor outputs break many assumptions in our tests:

1. Remove some hardcoded numbers in favor of a fee calc, so we only have to
   change in one place.
   FIXME: This should also be done for elements!
2. Do binary search to get feerate for a given closing fee.
3. Don't assume output #0: anchor outputs perturb them.
4. Don't assume we can make 1ksat channels (anchors cost 660 sats!).

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

tests/test_misc.py

@@ -2402,7 +2402,7 @@ def test_listtransactions(node_factory):
     """Sanity check for the listtransactions RPC command"""
     l1, l2 = node_factory.get_nodes(2, opts=[{}, {}])
 
-    wallettxid = l1.openchannel(l2, 10**4)["wallettxid"]
+    wallettxid = l1.openchannel(l2, 10**5)["wallettxid"]
     txids = [i["txid"] for tx in l1.rpc.listtransactions()["transactions"]
              for i in tx["inputs"]]
     # The txid of the transaction funding the channel is present, and