If we don't do this, when doing:
self.config_stub.val.content.user_stylesheets = css_path
then _update_stylesheet gets called before the stylesheet QWebEngineScript did
run (as there was no load yet), so we get:
[:2] Uncaught TypeError: Cannot read property 'stylesheet' of undefined!
Instead, load the page first and then update the stylesheet.
This tests that live updating works properly, and also makes sure we don't run
into the problem described above.
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