message: test: increase listbanned unit test coverage
  Add test coverage for the new ban_duration and time_remaining fields.
  While here, some code improvements.
diff: @@ -273,9 +273,9 @@ BOOST_AUTO_TEST_CASE(rpc_ban)
  ar = r.get_array();
  o1 = ar[0].get_obj();
  adr = find_value(o1, "address");
  - UniValue banned_until = find_value(o1, "banned_until");
  + int64_t banned_until{find_value(o1, "banned_until").get_int64()};
  BOOST_CHECK_EQUAL(adr.get_str(), "127.0.0.0/24");
  - BOOST_CHECK_EQUAL(banned_until.get_int64(), 9907731200); // absolute time check
  + BOOST_CHECK_EQUAL(banned_until, 9907731200); // absolute time check
  BOOST_CHECK_NO_THROW(CallRPC(std::string("clearbanned")));
  @@ -284,11 +284,16 @@ BOOST_AUTO_TEST_CASE(rpc_ban)
  ar = r.get_array();
  o1 = ar[0].get_obj();
  adr = find_value(o1, "address");
  - banned_until = find_value(o1, "banned_until");
  + banned_until = find_value(o1, "banned_until").get_int64();
  + const int64_t ban_created{find_value(o1, "ban_created").get_int64()};
  + const int64_t ban_duration{find_value(o1, "ban_duration").get_int64()};
  + const int64_t time_remaining{find_value(o1, "time_remaining").get_int64()};
  + const int64_t now{GetTime()};
  BOOST_CHECK_EQUAL(adr.get_str(), "127.0.0.0/24");
  - int64_t now = GetTime();
  - BOOST_CHECK(banned_until.get_int64() > now);
  - BOOST_CHECK(banned_until.get_int64()-now <= 200);
  + BOOST_CHECK(banned_until > now);
  + BOOST_CHECK(banned_until - now <= 200);
  + BOOST_CHECK_EQUAL(ban_duration, banned_until - ban_created);
  + BOOST_CHECK_EQUAL(time_remaining, banned_until - now);
  // must throw an exception because 127.0.0.1 is in already banned subnet range
  BOOST_CHECK_THROW(r = CallRPC(std::string("setban 127.0.0.1 add")), std::runtime_error);
  