Update Web MR tests to latest page behavior

Closes #447 (closed)

MR updates the Web MR tests to follow the pages current behavior and mimic tab behavior more correctly:

• All test now use latest params (and confirmed to be backwards compatible)
• web_project_merge_request_discussions is now just called web_project_merge_request. Along with discussions, this "main" page test now covers all the common calls the MR page makes.
• web_project_merge_request_commits now only covers calls specific to the Commits tab to allow focused testing on those endpoints.
• web_project_merge_request_changes now only covers calls specific to the Changes tab
  • Calls for diffs_batch have changed to increment in a set way instead of being a static amount each time.

More details for this change can be found in the issue.