Automate the basic API tests in a QA scenario

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6016

What does this MR do?

This automate basic API tests that:

1. Create a project
2. Create a file in the repository of the project
3. Get the file content
4. Delete the file
5. Get the tree
6. Delete the project

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

I don't think so.

Why was this MR needed?

This replaces the manual steps under "Basic API tests" here: https://docs.google.com/spreadsheets/u/1/d/1oy5ZtFyT7O4xR02Go0lPyhB5ar0L-Pg7tLVSks8oJ2Q/edit#gid=0

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-qa#251 (closed).