unit-tests: Seed chunk_hash() only once, but before creating any hashtables

Due to the removal of pthread_once, we manually create the seed for
chunk_hash(). With the new testable functions interface, this won't work for
the hashtable initiated using __attribute__((constructor)). Enforce seeding
before creating that hashtable.

src/libstrongswan/utils/chunk.h

@@ -343,7 +343,8 @@ bool chunk_printable(chunk_t chunk, chunk_t *sane, char replace);
  * Seed initial key for chunk_hash().
  *
  * This call should get invoked once during startup. This is usually done
- * by calling library_init().
+ * by calling library_init(). Calling it multiple times is safe, it gets
+ * executed just once.
  */
 void chunk_hash_seed();