Use 71 byte signature for DUMMY_SIGNATURE_CREATOR

Summary:
Changes DUMMY_SIGNATURE_CREATOR to create 71 byte dummy signatures.

Update comments to reflect this change

This is a partial backport of Core PR13666 : https://github.com/bitcoin/bitcoin/pull/13666/commits/48b1473c898129a99212e2db36c61cf93625ea17

Depends on D4467

Test Plan:

make check
./test/functional/test_runner.py

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4468