(Re)use functional_test_utils in lightning_liquidity

Instead of lightning_liquidity using its own utils and
having to (re)write a ton of code for simple things like
opening a channel and other basic node operations, reuse
the code from lightning/functional_test_utils. This allows
lightning_liquidity to extend its coverage and support
real full end-to-end testing.

As a first example of usage, the LSPS2 test
invoice_generation_flow that only tested invoice creation
now pays the invoice, intercepts the HTLC, creates the
channel, and forwards the HTLC.

Previously, this was only possible in ldk-node (which has
one full end-to-end LSPS2 test), but that setup wasn't
ideal. With this change, LSPS2 can now be tested directly
in this repo as well.

Author: martinsaposnic