Use ctest to execute external JSON tests

[chfast]

Currently on CI we just execute e.g. evmone-statetest tests_dir, but this runs in a single thread. We can use ctest to wrap the test execution for 2 benefits:

• tests executed in parallel,
• XML test result output understandable by CircleCI.

We don't have to mix it in the main CMake. We can write or generate CTestTestfile.cmake files. See documentation: https://cmake.org/cmake/help/book/mastering-cmake/chapter/Testing%20With%20CMake%20and%20CTest.html#testing-using-ctest.