[CI] Rename notebooks_check to test_notebooks and add test_wheels variable

[ulupo]

The CI has become too heavy for all the following to run at the same time:

• testing the dev install;
• testing the wheels;
• testing the notebooks.

The main cause for this is the increased test coverage as well as the complexity of some of the new notebooks (particularly the MNIST classification one).

I am introducing a new test_wheels variable in Azure to exclude unit tests of the wheels. This should be particularly useful in PRs devoted only to the notebooks.