Merge #18975: test: Remove const to work around compiler error on xenial

050e2ee test: Remove const to work around compiler error on xenial (Wladimir J. van der Laan)

Pull request description:

  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;

ACKs for top commit:
  MarcoFalke:
    Tested ACK 050e2ee on xenial with clang version 3.8.0-2ubuntu4 (tags/RELEASE_380/final)
  fanquake:
    ACK 050e2ee - I see why we didn't hit this on master. We are installing the `clang-8` packages for the tsan job. However on the 0.20 branch we are still just installing `clang`, which is 3.8.

Tree-SHA512: 8a1d57289dbe9895ab79f81ca87b4fd723426b8d72f3a34bec9553226fba69f6dc19551c1f1d52db6c4b2652164a02ddc60f3187c3e2ad7bcacb0aaca7fa690a

Author: fanquake

src/test/validationinterface_tests.cpp

@@ -23,7 +23,7 @@ BOOST_AUTO_TEST_CASE(unregister_validation_interface_race)
     // Start thread to generate notifications
     std::thread gen{[&] {
         const CBlock block_dummy;
-        const BlockValidationState state_dummy;
+        BlockValidationState state_dummy;
         while (generate) {
             GetMainSignals().BlockChecked(block_dummy, state_dummy);
         }