lntest: use nextAvailablePort for miner and btcd backend

With the new btcd version we can specify our own listen address
generator function for any btcd nodes. This should reduce flakiness as
the previous way of getting a free port was based on just picking a
random number which lead to conflicts.
We also double the default values for connection retries and timeouts,
effectively waiting up to 4 seconds in total now.

lntest/btcd.go

@@ -93,6 +93,14 @@ func NewBackend(miner string, netParams *chaincfg.Params) (
 		return nil, nil, fmt.Errorf("unable to create btcd node: %v", err)
 	}
 
+	// We want to overwrite some of the connection settings to make the
+	// tests more robust. We might need to restart the backend while there
+	// are already blocks present, which will take a bit longer than the
+	// 1 second the default settings amount to. Doubling both values will
+	// give us retries up to 4 seconds.
+	chainBackend.MaxConnRetries = rpctest.DefaultMaxConnectionRetries * 2
+	chainBackend.ConnectionRetryTimeout = rpctest.DefaultConnectionRetryTimeout * 2
+
 	if err := chainBackend.SetUp(false, 0); err != nil {
 		return nil, nil, fmt.Errorf("unable to set up btcd backend: %v", err)
 	}