[QA] Discuss reducing duplication vs. reducing readability in E2E doc

The following discussion from gitlab-ce!28716 should be addressed:

• @mlapierre started a discussion: (+2 comments)

  I think the doc should discuss the trade-off between reducing duplication and reducing readability. This might be a good place for that.

  By using the @labels variable instead of the label string values, we can't as easily read the examples. Even though it's a short, simple example I still have to put in some effort to mentally substitute the variables for the values when reading the tests.

  I think this is why thoughtbot avoids using let: https://github.com/thoughtbot/guides/blob/master/style/testing/avoid_let_spec.rb (and we refer to their styleguide in ours)

The examples have been updated, but it would still be useful to give a counterexample of code that reduces duplication but also reduces readability as a consequence (and how to avoid that).