[testing]: run unit tests against both bitcoind v27 and v28 (RBF opt-in vs. full RBF)

[guggero]

Came up here: #9059 (review)

We might want to run the unit tests with opt-in RBF (bitcoind v27) as well as full RBF (bitcoind v28).

Parts of this commit might need to be reverted and/or changed to run dependent on the backend version: c156d17