itest: fix issues caused by the bump to lnd 0.15.4-beta

[ellemouton]

In https://github.com/lightninglabs/faraday/pull/152, the lnd version that faraday uses was bumped from v0.15.-0-beta to v0.15.4-beta. This then caused some of the faraday itests to break. Specifically:

1. The closeChannel method started to use a zero value fee. This has temporarily been solved by ignoring the used fee and instead using a hardcoded fee value. See 1 and 2
2. The second issue is that force closes no longer show up as confirmed in the itest. This could possibly due to the first issue? Perhaps the zero fee txs dont get accepted into the mempool?