Move test initialization code to some shared location.

[martun]

This code is copied in all the tests:

using dist_type = std::uniform_int_distribution<int>;
std::size_t test_global_seed = 0;
boost::random::mt11213b test_global_rnd_engine;
template<typename FieldType>
nil::crypto3::random::algebraic_engine<FieldType> test_global_alg_rnd_engine;

struct test_initializer {
    // Enumerate all fields used in tests;
    using field1_type = algebra::curves::pallas::base_field_type;
    using field2_type = algebra::curves::bls12<381>::scalar_field_type;
    test_initializer() {
        test_global_seed = 0;

        for (std::size_t i = 0; i < boost::unit_test::framework::master_test_suite().argc - 1; i++) {
            if (std::string(boost::unit_test::framework::master_test_suite().argv[i]) == "--seed") {
                if (std::string(boost::unit_test::framework::master_test_suite().argv[i + 1]) == "random") {
                    std::random_device rd;
                    test_global_seed = rd();
                    std::cout << "Random seed = " << test_global_seed << std::endl;
                    break;
                }
                if (std::regex_match(boost::unit_test::framework::master_test_suite().argv[i + 1],
                                     std::regex(("((\\+|-)?[[:digit:]]+)(\\.(([[:digit:]]+)?))?")))) {
                    test_global_seed = atoi(boost::unit_test::framework::master_test_suite().argv[i + 1]);
                    break;
                }
            }
        }

        BOOST_TEST_MESSAGE("test_global_seed = " << test_global_seed);
        test_global_rnd_engine = boost::random::mt11213b(test_global_seed);
        test_global_alg_rnd_engine<field1_type> = nil::crypto3::random::algebraic_engine<field1_type>(test_global_seed);
        test_global_alg_rnd_engine<field2_type> = nil::crypto3::random::algebraic_engine<field2_type>(test_global_seed);
    }

    void setup() {
    }

    void teardown() {
    }

    ~test_initializer() {
    }
};