Add test for temporary file removal and WITH HOLD cursor

Add test for temporary file removal and WITH HOLD cursor

This new test, added in 009_log_temp_files, checks that the temporary
files created by a WITH HOLD cursor are dropped at the end of the
transaction where the transaction has been created.

The portal's executor is shutdown in PersistHoldablePortal(), after for
example some forced detoast, so as the cursor data can be accessed
without requiring a snapshot.

Author: Mircea Cadariu <cadariu.mircea@gmail.com>
Discussion: https://postgr.es/m/0a666d28-9080-4239-90d6-f6345bb43468@gmail.com