Migrate test suite to end-to-end tests

[ggalmazor]

Fixes #34

This PR affects only to the tests codebase.

This PR introduces a new TestHttpServer class responsible for replaying HTTP fixture files and providing tools to make assertions about the HTTP request that this API client makes.

The biggest change this PR introduces is a change in the way we express expectations, by asserting through a RecordedRequest object. This is due to the limitations of running the JUnit assertions in the same thread that the server is running, which is not possible without putting an important amount of effort into it. As a consequence, testing has to be made following these general ideas:

• DNSimpleBaseTest provides a server and a client instance and ensures that there's no interference between test methods.
• Assertions about the request have to be made through the RecordedRequest object that server.getRecordedRequest() provides. This object includes the JSON payload, headers, path, and HTTP method information which allows any of the existing assertions to be made.
• server.stubFixtureAt(String path) can be used to configure the server to replay a fixture file as the response to requests.
  For now, it's a basic stubbing system that will replay the same one fixture for all requests, but it could be easily extended to replay sequences of fixtures for multiple requests or specific fixtures per requested path or HTTP method.
• In order to test exception throwing blocks, there's now a thrownException matcher that enables verifying the exception class and doing introspective testing as well. Example:

  @Test
  public void testDisableDnssecWhenNotEnabled() {
    server.stubFixtureAt("disableDnssec/not-enabled.http");
  
    assertThat(() -> client.domains.disableDnssec("1", "example.com"), allOf(
        thrownException(is(instanceOf(DnsimpleException.class))),
        thrownException(property(DnsimpleException::getStatusCode, is(428)))
    ));
  }

  DomainDnssecTest.testDisableDnssecWhenNotEnabled

[san983]

Did a quick first pass and looks good. Tests run ✅but wonder if we can keep them less verbose.

[weppos]

I am removing myself from review. I'll leave @san983 here. A second pair of eyes is probably not necessary, but if you need one maybe @jacegu or another team member with Java experience can assist.

[ggalmazor]

Tests run ✅but wonder if we can keep them less verbose.

Just realized that I had some System.out.println calls leftover from when I was working on the test server. I've pushed a commit to get rid of them. Now you should only see assertion failures

[san983]

After live-reviewingthis with @ggalmazor, this looks good 👍 ✅.

I raised some questions that can fit in the roadmap Guille is gonna draft. Thanks!!!