Restructure tests directory hierarchy (pytest)

[irenaby]

Pull Request Description:

Restructure pytest directory into two levels: first level is framework/common, second level is unit/integration.

Checklist before requesting a review:

• I set the appropriate labels on the pull request.
• I have added/updated the release note draft (if necessary).
• I have updated the documentation to reflect my changes (if necessary).
• All function and files are well documented.
• All function and classes have type hints.
• There is a licenses in all file.
• The function and variable names are informative.
• I have checked for code duplications.
• I have added new unittest (if necessary).

[ofirgo]

Why do we need "common" inside "common_tests/unit_tests"?

[irenaby]

removed

[ofirgo]

What's Mixin?

[irenaby]

A design patter that provides specific functionality via class. It's not used in this PR but you can do
class MyTest(MaybeSomeOtherClass, KerasFwMixin) and get the functionality. The purpose of inheritance here is just to inject functionality.