[backport#17357 1/2] tests: Move CaseInsensitiveEqual to test/util/str

Summary:
https://github.com/bitcoin/bitcoin/pull/17357/commits/85a34b1683233060c4500cfb8e3f5d84c8b8f0da

Partial backport of Core PR17357

Test Plan:

ninja check

make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6904