[mining] Add a test for TestCBlockTemplateEntry

Summary:
Add the skeleton of a test to ensure that the constructor is setting the expected
fields, and can be filled in later for more complicated logic which will be added.

Test Plan:

make check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3162