Improve JSON test to verify that code works with various fields order

[nuald]

Per RFC 7159:

An object is an unordered collection of zero or more name/value pairs

JSON parsers shouldn't assume the order of fields in the most of the use-cases (surely, it's possible to have the exceptions when the both JSON and its parser are in the controlled environment). As this project is not about micro-optimizations, but rather the usual code, it could be quite confusing for the average software developer that their code is broken when they parse totally valid JSON.

Therefore, it would be nice to improve tests to verify that the parser is order-insensitive. The following example demonstrates the approach:

  for (const padded_string &v : {
      "{\"coordinates\":[{\"x\":1.1,\"y\":2.2,\"z\":3.3}]}"_padded,
      "{\"coordinates\":[{\"y\":2.2,\"x\":1.1,\"z\":3.3}]}"_padded}) {
    auto left = calc(v);
    auto right = coordinate_t(1.1, 2.2, 3.3);
    if (left != right) {
      cerr << left << " != " << right << endl;
      exit(EXIT_FAILURE);
    }
  }