Add instruction to merge test-suite branch in ep. 2.3

[sjvrijn]

Related to #365

This PR is based on the comment by @douglowe:

It looks to me like the test-suite branch isn't merged into develop at the end of the Continuous Integration for Automated Testing episode. Perhaps the intention was to have the test-suite branch merged into develop at the end of this lesson instead?

[steve-crouch]

Also related to #376, which appears to have a different solution to the same issue if I understand it correctly. After discussing it with Aleks, we'll need to work out the best way forward after the material has transitioned to the new format which has become a priority. We'll recreate the PR after that. At least we're not short on solutions for this :)

[bielsnohr]

Okay, pulling all the threads together. First, see my comment on issue #365 . Critically, we do have a section in the lesson material that does merge the test-suite branch into develop, but it is buried at the bottom of Episode 2.4. Please leave your feedback on that comment because I am going to close this PR. My justification for closing without merging is that we already have a section on merging test-suite and because the pylint-ci mix-up will be handled in the other PR #376 . Thanks for the contribution regardless!