common: fix run tests by superuser

`chmod -w` does not prevent a user with root privileges from creating a file in such directory. So, the test fails, as it expects *Permission denied* error.
The alternate solution would be to use i.e. `/proc` directory, as it guarantees that even the supersuer cannot create a file there. Similar approach for *util_pool_open*.
We can also remove the problematic test cases from unit tests...;-)