Fix dashboard class undefined

[cconard96]

Checklist before requesting a review

• I have read the CONTRIBUTING document.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.

Description

On main, sometimes dashboards are not loading and the GLPIDashboard JS global is undefined at the time the constructor tries being called. Added bare-minimum E2E test to check that dashboards are loading on each of the 4 included dashboard locations.

[AdrienClairembault]

Seems like it causes the search/display_preferences.cy.js test to fail, even after multiple tries it does not go green.

[cconard96]

Seems like it causes the search/display_preferences.cy.js test to fail, even after multiple tries it does not go green.

I noticed, even if I don't understand how they are related at all. This test seems to fail a lot even if it manages to pass after a few tries in other PRs. In this run:

1. Failed because it couldn't find #tabspanel-select
2. No failure noted. Should have been a pass?
3. Couldn't find Pending reason option (probably because it was already in the shown items list)
4. Same as failure 3

For the first failure, it is probably a matter of waiting for a network request using waitForNetworkIdle. For the last 2, I don't see the relation with this PR.