Initial tests and changes for performance tests (Comments in MR page)

This is the initial set of test to measure performance metrics for MR page.

Closes https://gitlab.com/gitlab-org/quality/team-tasks/issues/54

This MR does the following:

• Generates test data to test the comment action on MR page - specifically, creates an MR with 20 files with ~20K additions and deletion.
• Has test code to add comment to MR page (5 times - this is just an arbitrary count) and calculates the apdex_score based on this
• Calculates the page_load_time basedon windows.performance.timing
• The page_load_threshold and the response_threshold are again the average of the observed metrics.