tests: close a leftover file descriptor in `test-fileio`

tests: close a leftover file descriptor in `test-fileio`

This should make it a bit easier to search for real file descriptor leaks.

```
$ valgrind --leak-check=full --track-fds=yes ./build/test-fileio
...
==29457==
==29457== FILE DESCRIPTORS: 4 open at exit.
==29457== Open file descriptor 3: /tmp/test-systemd_writing_tmpfile.lyV5Rc
==29457==    at 0x4B9AD9E: open (open.c:43)
==29457==    by 0x4B19B24: __gen_tempname (tempname.c:261)
==29457==    by 0x4BA5CC3: mkostemp64 (mkostemp64.c:32)
==29457==    by 0x48F739B: mkostemp_safe (fileio.c:1206)
==29457==    by 0x10D968: test_writing_tmpfile (test-fileio.c:620)
==29457==    by 0x10E930: main (test-fileio.c:767)
==29457==
```