Test suite should clean up after itself

[workingjubilee]

Normally, TempDir deletes the directory you made with it when you drop it. This crate uses a Lazy<TempDir> for its tests so that parallel access means the directory is initialized the moment it is needed. However, it is then held in a static TEMP_DIR which means it is simply fixed in memory for the lifetime of the program. This tends to result in the Drop implementation for TempDir not executing properly... accumulating gigabytes of test output shockingly quickly and filling up drives. Either pgx-tests or plrust need to learn how to clean up after themselves.