Added an explicit yield (OP_SLEEP) to QUIC testing for cooperative th…

…reading.

Fixes: #24442

Signed-off-by: Randall S. Becker <randall.becker@nexbridge.ca>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Paul Dale <ppzgs1@gmail.com>
(Merged from #24443)

(cherry picked from commit b9e084f)

test/quic_multistream_test.c

@@ -2319,6 +2319,7 @@ static const struct script_op script_10[] = {
 static const struct script_op script_11_child[] = {
     OP_C_ACCEPT_STREAM_WAIT (a)
     OP_C_READ_EXPECT        (a, "foo", 3)
+    OP_SLEEP                (10)
     OP_C_EXPECT_FIN         (a)
 
     OP_END