Flaky frontend test in cypress

It looks like the "large screen sidebar collapse" test in cypress is flaky.

Example of failed build: https://jenkins.softwareheritage.org/job/DWAPPS/job/tests/1231/testReport/junit/(root)/should%20have%20less%20width%20when%20collapsed%20compared%20to%20open/Tests___Cypress_tests___Run_cypress_tests___Sidebar_tests_On_Large_Screen_should_have_less_width_when_collapsed_compared_to_open/

I'm guessing there's a race condition and the check doesn't wait for the sidebar to actually collapse, but I haven't looked yet.

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Migrated from T2341 (view on Phabricator)