E2E test: wait for file to load for all ide specs

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/471957

Occasionally ide specs fail as the project hasn't completely loaded, with this MR we wait for a file to be displayed for all ide e2e tests

In https://gitlab.com/gitlab-org/gitlab/-/issues/471957 we see in the screenshot that project files aren't yet loaded, even though our wait_for_ide_to_load method has completed. This adds an additional implicit wait to wait for a project file also.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

See https://gitlab.com/gitlab-org/gitlab/-/issues/471957

How to set up and validate locally

It affects most ide specs, so probably best to look at pipeline results.