Add test for making sure jump to next discussion works

This is a follow up gitlab-ce~3011693 from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13076

I attempted to create test for that simple gitlab-ce~3630472 fix but hit a few roadblocks along the way.

• This test cannot be done in rspec because rspec loads the entire view (hence scrolling is unavailable)
• I attempted to do an integrated karma test by generating the fixture for the MR page but @click was being parsed out by nokogiri when the fixture was generated (as determined in a call with @jschatz1, @winh and @DouweM)

Next steps:

• @winh is going to see if we can change the fixture generator to not remove @click. We currently use nokogiri to parse the script tags, it seems like nokogiri may have some options we can pass in so that element attributes remain intact. The other option, is to use regex to remove the script tags.
• After we are able to generate the fixtures with @click, we can attempt to create the integrated test (which may also be quite challenging due to the structure of this vue app)