[INTEGRATION][AIRFLOW] Clean up unnecessary code in tests for extractors

[sekikn]

Signed-off-by: Kengo Seki sekikn@apache.org

Problem

There are some weird things in the unit test for PostgreExtractor.

https://github.com/OpenLineage/OpenLineage/blob/0.7.1/integration/airflow/tests/extractors/test_postgres_extractor.py#L130-L131
The test_extract method doesn't need this environment variable, since it mocks get_connection instead.
This variable is actually required by test_get_table_schemas, so it should be moved inside there.

https://github.com/OpenLineage/OpenLineage/blob/0.7.1/integration/airflow/tests/extractors/test_postgres_extractor.py#L192-L194
This hook isn't used anywhere, so it can be removed.

We can also find similar problems in the tests for SnowflakeExtractor and MySqlExtractor (because I wrote the latter by copy-and-pasting from PostgreExtractor...).

Solution

This PR moves and deletes the code explained above.

Checklist

• You've signed-off your work
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• Your changes are accompanied by tests (if relevant)
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• You've updated any relevant documentation (if relevant)
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