Add testable date times

[robsonsilv4]

Description

This PR replaces untestable DateTime.now() with clock.now() from pkg:clock.
With this change, the package and the clients of the package could be testing token date times using pkg:fake_async.

And the best part, this change doesn't add breaking changes or change the current behaviors!

Here's a sample of testing an expired token:

import 'package:dart_jsonwebtoken/dart_jsonwebtoken.dart';
import 'package:fake_async/fake_async.dart';
import 'package:test/test.dart';

void main() {
  test('should expires when `expiresIn` duration is elapsed', () {
    final secretKey = SecretKey('secret_key');
    final jwt = JWT({'foo': 'bar'});
    final duration = Duration(hours: 1);
    fakeAsync((async) {
      final token = jwt.sign(secretKey, expiresIn: duration);
      async.elapse(duration);
      expect(
        () => JWT.verify(token, secretKey),
        throwsA(isA<JWTExpiredException>()),
      );
    });
  });
}

Note: clock and fake_async are published by Dart Team:

• https://pub.dev/publishers/tools.dart.dev/packages
• https://pub.dev/publishers/dart.dev/packages

[robsonsilv4]

@jonasroussel I tried to follow your pattern of tests. Let me know if you need to change anything.

[robsonsilv4]

CC: @alestiago

[jonasroussel]

Hey @robsonsilv4,

Thanks for your changes! I just refactor your tests to put them in another test file so I can evolve the header tests in the future, I made those changes in another branch here: https://github.com/jonasroussel/dart_jsonwebtoken/tree/testable-datetime. I'll let you merge it into your PR repo branch and then I'll approve it.

[robsonsilv4]

Already merged, thanks @jonasroussel
It's really better to split the tests before they grow more