test: Remove const to work around compiler error on xenial

Summary:

Fix the following error in travis:

test/validationinterface_tests.cpp:26:36: error: default initialization of an object of const type 'const BlockValidationState' without a user-provided default constructor

const BlockValidationState state_dummy;

This is a backport of Core PR18975

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9116