QUIC MULTISTREAM TEST: Add comment

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21827)

test/quic_multistream_test.c

@@ -557,6 +557,10 @@ static unsigned int server_helper_thread(void *arg)
 
         ossl_quic_tserver_tick(h->s_priv);
         ossl_crypto_mutex_unlock(h->server_thread.m);
+        /*
+         * Give the main thread an opportunity to get the mutex, which is
+         * sometimes necessary in some script operations.
+         */
         OSSL_sleep(1);
         ossl_crypto_mutex_lock(h->server_thread.m);
     }