Move tests outside the application code folder

[tatiana]

Currently, the tests are together with the application code.

The goal of this ticket is to move them to their own directory tests in the root, having a similar folder directory structure to the application code.

Some motivations:

• The Python Packaging Authority recommends separating the test directory from the application code (src)
• We are not distributing tests with the application, and we have tests which do not match 1:1 files, such as the integration example DAGs tests (what PyTest says about structure)
• This is a pattern established not only in Airflow and many major Python projects but also in most (if not all) Astronomer Python projects, including Astro SDK, Multitenancy, providers packages and ​​even our template repository)