Add E2E for working with forks in Web IDE

What does this MR do?

Adds E2E coverage for working with forks in Web IDE. This MR covers the first two scenarios from gitlab-org/quality/testcases#923 (closed):

User has no permissions to commit to the project

• no fork created => suggested to create a fork on opening Web IDE => can create MR upstream
• fork present => button changes to "Edit fork in Web IDE" and fork is opened on click => can create MR upstream

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content