fix(e2e): wait for tab in e2e code suggestions spec

Description

Fixes flakiness in e2e code suggestions spec by waiting for the editor tab to appear before trying to interact with it.

Related Issues

Resolves #1344 (closed)

How has this been tested?

Locally and in pipelines https://gitlab.com/gitlab-org/gitlab-vscode-extension/-/jobs/6801589211

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap