Fix unable to expand text diff discussion comments

What does this MR do?

• Fixes a bug whereby text diff discussions that were resolved wouldn't expand after the page was reloaded. This was caused by the collapsed CSS class being applied to all discussions which was hiding it from text diffs. It worked for image diffs because image diffs have a separate toggle inside of that haml partial.
• Reverted bad spec (https://gitlab.com/gitlab-org/gitlab-ce/blob/82237c688faf2633d4a1a349d9462f0be6754a12/spec/features/merge_requests/diffs_spec.rb) that caused this regression to get missed

How did this regress?

• @ClemMakesApps forgot to check this state
• @felipe_artur accidentally changed the spec that tested the correct state to test an incorrect state (which then passed)

Are there points in the code the reviewer needs to double check?

Shouldn't be

Why was this MR needed?

Fixes regression

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Frontend

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/38940