After unit testing, use `grn_fin (n)` in pgroonga.log to detect memory leaks

### What is your problem?

GH-939 was found by checking `pgroonga.log` after running the unit tests.
We found `grn_fin (1)` in the log.
So, how about checking for `grn_fin (n)` in pgroonga.log after the unit tests?

### How to reproduce it

_No response_