fuzz: create tmpfiles in /tmp

fuzz: create tmpfiles in /tmp

It's mostly a cosmetic change that should prevent the fuzzer
from cluttering the "$OUT" directory (which OSS-Fuzz uses to
build docker images):

```
Step #44: Already have image: gcr.io/oss-fuzz/lxc
Step #44:   adding: fuzz-lxc-config-read (deflated 67%)
Step #44:   adding: fuzz-lxc-config-read-WBWKxN (deflated 32%)
Step #44:   adding: fuzz-lxc-config-read_seed_corpus.zip (stored 0%)
Step #44:   adding: honggfuzz (deflated 66%)
Step #44:   adding: llvm-symbolizer (deflated 65%)
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>