I work on PGlite [0], integration testing is absolutely something we want it to be useful for. For an early data point the Drizzle integrations tests run in about 1/4 the time with PGlite in-memory than a full Postgres.
The OP mentions lack of extensions being a blocker for them using it now, they are coming soon. Additionally it's currently limited by being single connection, and again I hope we can remove this limitation.
I also have a few other ideas around how to make supper fast testing possible.