BIP 54 test vectors improvements following review in Inquisition

This addresses the feedback received on the test vectors following the merge of #2015, the review of the Bitcoin Inquisition implementation (https://github.com/bitcoin-inquisition/bitcoin/pull/99), and Chris Stewart's implementation for Bitcoin-s (https://github.com/bitcoin-s/bitcoin-s/pull/6170).

There are 4 categories of changes here:

• Rewording and clarifications in the BIP text.
• Some typo fixes and field reordering in the test vectors.
• A change in the structure of the timestamps test vectors, to reduce the size of the file by ~80%.
• Test case addition(s).

See commit messages for details.