Added support for NBTest, for testing notebooks with assertions in the `Multi-software test` CI pipeline

[vrnimje]

Closes #1761

Hi, We are developers of the NBTest framework. We are following up on the previous discussion #1761. We can now use NBTest's assertions directly in the notebooks which were part of your library's tests. Hence, we have replaced nbval with nbtest in the CI. Please let us know if these changes make sense. We are happy to answer any questions! Thanks!

Description of the changes:

• Modified all the notebooks in the tests/notebooks/ directory to utilise NBTest assertions
• Replaced --nbval flag with --nbtest in the Makefile
• Reordered the test steps to demonstrate NBTest's pytest plugin in the CI (which were not executed due to failing unit tests)
• Updated pyproject.toml to include nbtest-gen as it's test dependency, instead of nbval

Note: This notebook has some dataset API errors, but the rest of them do work as expected.

[vrnimje]

@fabclmnt Please review