test: Fix memory leak of asynctest

ASYNC_init_thread() will be called automatically by ASYNC_start_job(),
so ASYNC_cleanup_thread() must be called at last, otherwise it will
cause memory leak.

Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16703)

test/asynctest.c

@@ -411,6 +411,7 @@ static int test_ASYNC_start_job_ex(void)
     ret = 1;
  err:
     ASYNC_WAIT_CTX_free(waitctx);
+    ASYNC_cleanup_thread();
     OSSL_LIB_CTX_free(libctx);
     return ret;
 }