gitlab-qa !155 - push creates event on activity page

What does this MR do?

Implement scenario to ensure that a push creates an event in the activity page (related to issue gitlab-qa#155 (closed))

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

I'm having issues raising GDK on my local machine, haven't been able to run the test against a Gitlab (GDK) instance. When I run against Gitlab docker image, the qa specific selectors are not there. Solved in the context of gitlab-development-kit#311 (closed)

Why was this MR needed?

Test the result of a successful PostReceive (instead of looking into Sentry), from the end-user perspective.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

gitlab-qa#155 (closed)

/cc @rymai