Integration Testing Guide

We should have a guide for adding integration tests to a Dash repo.

Right now the only real resource is reading the previous code and figuring it out.

A few points I think this guide should have, let me know if there are more!

1. Installing Selenium and the Chrome Driver tools. I have a gist of that process here that we could modify.
2. Intro to the testing lifecycle methods setUpClass, tearDownClass, setUp, tearDown so a user understands what happens before each test, after each test, once before all the tests, and once after all the tests
3. What is percy and how to take percy screenshots
4. Simple example of a test (https://github.com/plotly/dash/blob/f309cc2b8a45644004db98ffcaf0d9a72870c27d/tests/test_integration.py#L24)
5. Links to selenium resources (http://selenium-python.readthedocs.io/), although I really hate these docs and find it more helpful to just read the code (https://github.com/SeleniumHQ/selenium/tree/master/py). Maybe we can
6. Link to https://docs.python.org/3.6/library/unittest.html#module-unittest, maybe a separate list of the most useful self.assert... functions.

[gwdekker]

Great idea! Would love to have this for pytest as well, not only unitTest.

[byronz]

most of the discussion in this issue is included in https://dash.plot.ly/testing