Add waiting for requests after posting a comment in commit_spec.rb

What does this MR do and why?

This MR adds wait_for_requests after the click to post a comment. The test would occasionally fail because, prior to this fix, the assertion was made without first waiting for "My comment" to be present, and instead used a discussion that is above that comment.

Related to #386199 (closed) and gitlab-org/quality/engineering-productivity/master-broken-incidents#454 (closed)

Sample failure

1) Thread Comments Commit behaves like thread comments for commit and snippet clicking "Comment" will post a comment
  Got 1 failure and 1 other error:
  Shared Example Group: "thread comments for commit and snippet" called from ./spec/features/discussion_comments/commit_spec.rb:21
  1.1) Failure/Error: expect(new_comment).not_to have_selector '.discussion'
          expected not to find visible css ".discussion" within #<Capybara::Node::Element tag="li" path="/HTML/BODY[1]/DIV[3]/DIV[1]/DIV[3]/MAIN[1]/DIV[2]/DIV[7]/UL[1]/LI[1]">, found 1 match: "Sidney Jones5\n@namespace3 started a thread just now\nLast updated by Sidney Jones6 just now\nHide thread\nSidney Jones5 @namespace3\n· just now\nOwner\nMy title 3\nSidney Jones6 @namespace4\n· just now\nMy title 4\nReply..."
          Timeout (30s) reached while running a waiting Capybara finder.

How to set up and validate locally

Run bundle exec rspec 'spec/features/discussion_comments/commit_spec.rb[1:1:1]' in this branch.

MR acceptance checklist

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