bitcoin: If we fail to estimate the fee in testnet use the minfee

When developing in regtest or testnet it is really inconvenient to
have to fake traffic and generate blocks just to get estimatesmartfee
to return a valid estimate. This just sets the minfee if bitcoind
doesn't return a valid estimate.

Reported-by: Rene Pickhardt <@renepickhardt>
Signed-off-by: Christian Decker <@cdecker>

tests/test_misc.py

@@ -864,6 +864,7 @@ def test_ipv4_and_ipv6(node_factory):
         assert int(bind[0]['port']) == port
 
 
+@unittest.skipIf(not DEVELOPER, "Without DEVELOPER=1 we snap to FEERATE_FLOOR on testnets")
 def test_feerates(node_factory):
     l1 = node_factory.get_node(options={'log-level': 'io'}, start=False)
     l1.daemon.rpcproxy.mock_rpc('estimatesmartfee', {