tests: Test that a fully signed tx given to signrawtx is unchanged

Summary:

Tests that a fully signed transaction given to
signrawtransactionwithwallet is both unchanged and marked as complete.
This tests for a regression in Bitcoin Core 0.20 where the transaction would not be
marked as complete.

We do not have the regression in Bitcoin ABC and it seems unlikely that it will happen, as it was bissected to core#16528 which we already finished backporting, and had to be fixed only on Core 0.20 (not master). But just in case, it is better to have the test to avoid any bad surprise.

This is a backport of core#20562

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10894