test: rm ascii art in rpc_fundrawtransaction

Summary:

Doc changes only to test/functional/rpc_fundrawtransaction.py:

• remove ascii art or convert to a docstring when sufficiently different from the logging
• touch up other comments while here

This is a backport of Core PR17342 [2/2]
https://github.com/bitcoin/bitcoin/pull/17327/commits/ff22751417c6fbbd22f4eefd0e23431a83335c13

Depends on D8234

Test Plan:
ninja && test/functional/test_runner.py rpc_fundr*
(just in case any typo introduced a SyntaxError)

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8235