[CMAKE] Make the list of tests a property of the test suite

Summary:
Unused at the moment, this will allow to read back the list of tests
from a test suite.

There is no change in behavior.

Depends on D5311.

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5312