[improve][tests] Make language function integration test classes abstract

[cbornet]

Motivation

The tests from these classes are only executed from their subclasses.
In IntelliJ IDEA, if the class is not abstract, when you launch the test with the ▶️ button, it fails saying that there are no tests to execute.
If the class is abstract, when you launch the test with the ▶️ button, it asks for which subclass you want to run the test : all, ThreadRunTime, ProcessRuntime.

Modifications

trivial

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

no

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

Documentation

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  tests

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