Migrate browser performance widget specs from `ee_mr_widget_options_spec.js` to `grouped_browser_performance_reports_app_spec.js`

Summary

Follow-up from !46746 (merged)

Move the test cases from the browser_performance section in ee_mr_widget_options_spec.js to grouped_browser_performance_reports_app_spec.js (created in !46746 (merged)) because ee_mr_widget_options_spec.js is large, slow, and hard to work with (see #214032)

Improvements

• Tests for the browser performance widget will be easier to find, understand, and update
• Tests for each widget can run faster if we run them individually in parallel instead of in one large file
• Tests for one widget will be less likely to cause hard-to-debug side effects on tests for other widgets

Risks

Modifying the specs for the web/browser performance MR widget

Involved components

• grouped_browser_performance_reports_app_spec.js
• ee_mr_widget_options_spec.js