Fix activation_tests

Summary:
The check for the null case was removed in a previous patch. This is a mistake as nullptr is passed down when checking for activation for the genesis block - it has no parent.

Also change an assert into a BOOST_CHECK_EQUAL for better error report.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4585