Add pytest fixtures to run dash.

[T4rk1n]

Availables fixtures:

• start_dash, start a dash app instance in a thread.
• dash_from_file, use runpy to execute a file from a path, return the dash app instance contained in that file.
• dash_app, combine dash_from_file and start_dash
• percy_snapshot, take a percy snapshot.

All these fixtures are functions that needs to be call from the tests.

Example:

def test_usage(dash_app, selenium):
    app = dash_app('usage.py')
    output = selenium.find_element_by_id('output')
    assert output == 'my-value'
    ...