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>

src/tests/fuzz-lxc-config-read.c

@@ -10,7 +10,7 @@
 
 int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
 	int fd = -1;
-	char tmpf[] = "fuzz-lxc-config-read-XXXXXX";
+	char tmpf[] = "/tmp/fuzz-lxc-config-read-XXXXXX";
 	struct lxc_conf *conf = NULL;
 
 	fd = lxc_make_tmpfile(tmpf, false);