Fix spec violations in spec/frontend/projects/compare/components/revision_dropdown_legacy_spec.js

Problem description

The spec file spec/frontend/projects/compare/components/revision_dropdown_legacy_spec.js contains technical debt and/or antipatterns that need to be fixed. This is a prerequisite for updating our codebase to Vue3.

Proposal

The following issues have been identified in this spec file:

• .setData() usage

Please see the related epic for more details on how these violations can be addressed.