Include axe checks in `spec/features/dashboard/merge_requests_spec.rb` feature test

Accessibility tests not detected. Refer to https://docs.gitlab.com/ee/development/fe_guide/accessibility/automated_testing.html#separate-file-for-extensive-test-suites on how to include them.

Url to the view in question: https://gitlab.com/dashboard/merge_requests?assignee_username=masked_assignee_username