Improving `testEndpoint`

[RobinTail]

Based on discussion #1799

During the time of testEndpoint exists I've got 3 feedback from users struggling to set it up correctly or being confused by the way it's organized. I'd like to make it easier to use due to that fact.

I'm going to change the way testEndpont() works. Instead of using mocking functions from testing frameworks, I will use the library that mocks request and response completely.

• This enables a fully functional request object.
• This affects the way assertions on response should be made: using new methods _getStatusCode(), _getData() and _getHeaders().
• Logger mocking will be also changed for consistency.

[coveralls-official[bot]]

coverage: 100.0%. remained the same when pulling 9180d4819c5443293c42a5404c4086aef1a58e90 on impr-testing-mocks into 2c683377ad9b5bc1250382e0818eef72dc32df2f on prep-v20.

[RobinTail]

This should be breaking and address v20

[RobinTail]

Changed the base

[RobinTail]

Need to update Readme and Changelog