test: Remove const to work around compiler error on xenial
test: Remove const to work around compiler error on xenial

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;

Github-Pull: #18975
Rebased-From: 050e2ee6f28e7b31c167013be7313726e34084e9

monacoinproject/monacoin,GroestlCoin/bitcoin,GroestlCoin/bitcoin,GroestlCoin/GroestlCoin,vertcoin/vertcoin,DigitalPandacoin/pandacoin,GroestlCoin/bitcoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,vertcoin/vertcoin,qtumproject/qtum,GroestlCoin/GroestlCoin,monacoinproject/monacoin,qtumproject/qtum,monacoinproject/monacoin,ahmedbodi/vertcoin,peercoin/peercoin,qtumproject/qtum,ahmedbodi/vertcoin,GroestlCoin/bitcoin,ahmedbodi/vertcoin,vertcoin/vertcoin,GroestlCoin/GroestlCoin,peercoin/peercoin,GroestlCoin/bitcoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,monacoinproject/monacoin,GroestlCoin/GroestlCoin,qtumproject/qtum,qtumproject/qtum,monacoinproject/monacoin,GroestlCoin/GroestlCoin,ahmedbodi/vertcoin,vertcoin/vertcoin,ahmedbodi/vertcoin,qtumproject/qtum,peercoin/peercoin,GroestlCoin/GroestlCoin,peercoin/peercoin,vertcoin/vertcoin,vertcoin/vertcoin,GroestlCoin/bitcoin,peercoin/peercoin,DigitalPandacoin/pandacoin,monacoinproject/monacoin,qtumproject/qtum,ahmedbodi/vertcoin