[CMAKE] Factorize the test suite target name construction

Summary:
This is a minor refactor that adds a facility for building a test suite
target name from the test suite name.

There is no change in behavior.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5311