add DummyAlertAggregator and change test_basic_pipelines.

[ernstleierzopf]

Task

Fixes #172

Description

• add DummyAlertAggregator
• add DummyAlertAggregator to test_basic_pipelines.py
• finding: lines such as assert (result := alert_aggregator.process(data)) is not None produce the unintended side effect of calling alert_aggregator.process(data) twice. This is not intended and I changed all occurrences I found in the tests. (tested by setting log level to DEBUG). - see Walrus expression duplicate evaluation failures with rewrite pytest-dev/pytest#14445

How Has This Been Tested?

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.

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