Add additional tests for pytest_itde

[ckunki]

This ticket was triggered by a discussion on PR #354 and requests to enhance the tests for ITDE's pytest plugin.

Proposal: The test should

• get a session wide database with random ports
• verify ability start new databases with random ports

Rationale: using default ports in tests is in general somewhat dangerous.

Please review / fix grammar of the following statement:

In that case, we only need port CLI parameter, in case we want to supply a database from the outside.

[Nicoretti]

pytest plugin isn't part of this repository anymore