Add fudament for API based on connexion

[mik-laj]

Close: #8109

---

Make sure to mark the boxes below before creating PR: [x]

• Description above provides context of the change
• Unit tests coverage for changes (not needed for documentation changes)
• Commits follow "How to write a good git commit message"
• Relevant documentation is updated including usage instructions.
• I will engage committers as explained in Contribution Workflow Example.

---

In case of fundamental code change, Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in UPDATING.md.
Read the Pull Request Guidelines for more information.

[turbaszek]

This class code is repeated in each test file. Also we have http_client fixture which also provides the app 😸

[mik-laj]

Yes. There are so many repetitions in this code because it is a skeleton. I hope that soon this code will be more different. I didn't want to introduce 3-level inheritance here, because it would complicate the tests excessively. This is ok if the code is repeated because it makes the code much simpler and less complex. Now each test file is not dependent on anything and each person can work independently.
Here is an article about it: https://testing.googleblog.com/2019/12/testing-on-toilet-tests-too-dry-make.html

[mik-laj]

Here is a real example: https://github.com/apache/airflow/blob/69c546fb0fa779f2f311e5a96c98900ef997afce/tests/api_connexion/endpoints/test_dag_model_endpoint.py

[turbaszek]

And what about the fixture? Should it stay or not?

[mik-laj]

I removed fixture: 6c765df
Thanks.

[potiuk]

LGTM