[backport#17342][tests] Don't use nScriptCheckThreads in the checkqueue_tests

Summary:
It's only needed for a hardcoded int, which we can define locally.

Note from Backporter: Core's description of this is indeed somewhat confusing. nScriptCheckThreads is removed in the next commit.

https://github.com/bitcoin/bitcoin/pull/17342/commits/d9957623b48a7c3eff0ac750d1245fabfb1843a2

Partial backport of Core PR17342

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7274