If the in-memory DB has the same feature set as the real DB, using the in-memory DB for tests should be a single flip you switch, while writing code to generate fake result sets for every tests requires writing all of that code. 90% of the benefits for 1% of the cost.
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If the in-memory DB has the same feature set as the real DB, using the in-memory DB for tests should be a single flip you switch, while writing code to generate fake result sets for every tests requires writing all of that code. 90% of the benefits for 1% of the cost.
But they do not have the same feature sets. And PostgreSQL trivially be tuned to work almost like an in-memory database.