Move Web IDE assets specs from main project

MR:

• chore: Move package tests from main repo (!402 - merged)
• Remove specs that have moved to web-ide project (gitlab!178877 - merged)

Description

In Move security patch from main project to `gitla... (#356 - closed), the Web IDE package patch is moved from the main project to the gitlab-web-ide repo. We've also added a test to check for the post-install hook in Upgrade @gitlab/web-ide package (gitlab!177503 - merged) as it's the fastest way to add test coverage.

Let's move the related specs in /spec/frontend/ide/web_ide_assets_spec.js to the Web IDE so that it would be colocated next to the relevant changes.

Acceptance Criteria

• Move the web_ide_assets_spec test into this project.

• Maybe this is an "integration" test since we're not actually testing units... Maybe we should add a sibling to packages/ folder for tests/ which house our tests that require the whole project to be built.
• We'll need to make sure this test runs on CI.