Merge #10777: [tests] Avoid redundant assignments. Remove unused variables

Summary:
49eb0916d [tests] Avoid redundant assignments. Remove unused variables. (practicalswift)

Pull request description:

• Remove unused variables.
• Avoid redundant assignments.

Tree-SHA512: 74dd59680c785c601982a791648bc5d9b04e2b4509119e05f2e402b94998f5bbcbdea1f82d05e5f431fe06642e8db7c346fa8bc4c11f8cdcc554bc70d735534c

Backport of Core PR10777
https://github.com/bitcoin/bitcoin/pull/10777/

Test Plan:

make check
test_runnery.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3671