Add Flask-like test_client to replace sanic_endpoint_test

[subyraman]

This is a big PR but it is fully backwards compatible with previous code.

The sanic_endpoint_test method was incongruous with the Flask testing API, and also wasn't fully documented. Rework testing so that, similar to flask, you run tests against methods like app.test_client.get. The methods are essentially sugar around sanic_endpoint_test, so it preserves all the work done before.

From the docs I wrote:

The test_client exposes get, post, put, delete, patch, head and options methods
for you to run against your application. A simple example (using pytest) is like follows:

# Import the Sanic app, usually created with Sanic(__name__)
from external_server import app

def test_index_returns_200():
    request, response = app.test_client.get('/')
    assert response.status == 200

def test_index_put_not_allowed():
    request, response = app.test_client.put('/')
    assert response.status == 405

Ripped the bandaid in this PR by rewriting the tests to use the test_client as well.

Put in a deprecation notice that sanic_endpoint_test would be removed in version 1.0, can be changed if that's too aggressive.

[seemethere]

You can probably leave this to say the next major version after 0.4.0

[JordanP]

I had a quick look at this PR and coming from a Flask background this way of testing app seems more natural to me.