TEST: Replace hard-coded hex tx with classes

Summary:
This adds some abstraction and make the code more readable by replacing byte operations on hex transactions by CTransaction & CTxOut objects.
The PR mentions a minor 4% increase in time for some tests, which is considered an acceptable compromise in exchange for more clarity.

The imports are not at the top of the module to avoid a circular import issue. This indicates that the util and messages modules would benefit from some refactoring.

This is a backport of Core PR15282

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8378