Add E2E spec that navigates to the Merge Request Dashboard

Context

Corrective action from gitlab-com/gl-infra/production#20599 (closed).

The MR homepage did not load at all on production canary due to a deployment issue. This could have been caught earlier in staging canary if there was an E2E test for this page.

Goal

Add an E2E test that navigates to the merge request dashboard (/dashboard/merge_requests) and validates that some content is shown.