Use msg_witnessblock instead of msg_block in all functional tests

[scravy]

In https://github.com/dtr-org/unit-e/pull/798 only the minimally necessary adjustments were done to functional tests to make them pass. This leads to confusion sometimes when working on them as blocks seemingly arbitrary fail to validate when introducing new features.

Since all our blocks are witness blocks msg_block should be swapped completely in favor of msg_witness_block in functional tests.

[AM5800]

I think we better remove msg_witness_block. Because why make a distinction? It is a block. And we don't have non-witness ones

[scravy]

We can do that. For the sake of clarity when reviewing I would prefer to do it in two steps:

• Replace all msg_block with witness block (makes it clear which places had to be touched and what was affected)
• Rename msg_witness_block to msg_block (this latter step should be trivial then)

[scravy]

I did go with making it msg_block in one shot... #1029