Implement ci-dnf-stack/unit tests and documentation for Python API

[kontura]

Every function on dnf5 Python API should be documented and should have a test to verify that it works. The tests should be unit tests if possible or a CI tests if necessary based on the required setup for the given test.

In general we should first create documentation for a function and then based on it create the tests.

We need tests for:

libdnf5.base:

• Base
• Goal
• [Goal structures and enums]
• [QueryCmp]
• [LogEvent]
• Transaction
• [TransactionPackage]

[libdnf::advisory]:

• Advisory
• AdvisoryCollection
• AdvisoryModule
• AdvisoryPackage
• AdvisoryQuery
• AdvisoryReference
• AdvisorySet

libdnf5,repo:

• RepoQuery
• [RepoSack]
  • add missing API docs
  • don't forget to mention how variables are substituted when creating repos

libdnf5.rpm:

• Package
• [Checksum]
• PackageQuery
• [PackageSack]
• [PackageSet]

(The list will be continually updated and links should point to Python docs not cpp.)

[kontura]

Advisory docs and tests: #987