[Feedback] Conan Essentials - `test_package`

While the quiz at the end of the section had a question about relation of the test_package and unit tests - I feel like it should be pressed much harder in the course itself that:
1. Test_package is _not_ about unit tests.
2. It runs only once at the time of a recipe export, only for the current configuration.

Review the Conan Essentials section and the quiz question about test_package and unit tests. Clarify in the course text that test_package is not for unit tests and runs once during recipe export for the current configuration; verify that the explanation matches the quiz.

Written by the indexing model from the issue text.