Put the fuzzer realm file in the working directory

mkstemp() eventually becomes a bottleneck due to thousands of stale
files from crashes.

fuzzer/fuzzer.cpp

@@ -201,9 +201,7 @@ int main(int argc, char** argv) {
     realm::disable_sync_to_disk();
 
     Realm::Config config;
-    config.path = getenv("TMPDIR");
-    config.path += "/realm.XXXXXX";
-    mktemp(&config.path[0]);
+    config.path = "fuzzer.realm";
     config.cache = false;
     config.in_memory = true;
     config.automatic_change_notifications = false;