override fixture config for a specific test

In https://github.com/plotly/dash-core-components/pull/932 @AnnMarieW discovered a browser feature (the clipboard API) that fails in headless mode. We introduced a hacky solution to it there to override --headless that the tests run with on CI (or whenever you run via npm run test or npm run test:intg): a separate fixture that's identical to dash_dcc (which itself is a minor modification of dash_duo) except that it explicitly sets headless=False rather than reading it from request.config.

We should have a better way to do this - maybe there already is one, I just couldn't find it? I'd love to have a decorator something like:

@dash_config(headless=False)
def test_clp001_clipboard_text(dash_dcc):
    ...

Start by tracing the dash_dcc and dash_duo fixtures, especially how they read headless from request.config, and review the npm run test and npm run test:intg paths. Done means a per-test configuration mechanism can replace the separate fixture while allowing test_clp001_clipboard_text to run with headless=False.