Document how to test with PyTest

[svnv-svsv-jm]

Describe the Enhancement you want
A clear and concise description of what the improvement does.

• Which feature do you want to improve? (and what problem does it have)
  Documentation.

• What is the benefit of the enhancement?
  Document how one could write automated tests for reflex using PyTest. This would enormously benefit developers who rely on automated tests for their work.

Additional context
Automated tests are mandatory for PROD work. Currently, there is no recommended way to write them for reflex, meaning I can only create a very small UI for personal demos and fun. No way I am going to (only) run the app in a terminal, then automate UI interactions, etc.

We need a PyTest-able way to do so, that would also affect coverage, etc.

[linear]

ENG-5964 Document how to test with PyTest

[Alek99]

If you are talking about testing the app as a whole you probably would want playwright. Example here https://github.com/reflex-dev/reflex-web/tree/main/tests