Replace class "js-builds-dropdown-button" in favor of a data-testid selector

Description

Currently the .js-builds-dropdown-button class is only used in specs such as:

• spec/features/projects/pipelines/pipelines_spec.rb
• spec/frontend/mini_pipeline_graph_dropdown_spec.js
• spec/frontend/fixtures/static/mini_dropdown_graph.html
• spec/frontend/pipelines/pipelines_spec.js
• spec/frontend/pipelines/pipelines_table_row_spec.js

This class should be replaced by a [data-testid=...] style selector in our tests.