xtest: regression_6016: use 3 threads instead of 4

Since OP-TEE commit cda03b63cf30 ("Enable SHA-3 support by default") [1]
regression_6016 fails when OP-TEE OS is compiled for QEMUv8 with
"make CFG_ULIBS_SHARED=y CFG_WITH_PAGER=y":

 * regression_6016 Storage concurency
 o regression_6016.1 Storage id: 00000001
     threads: 4, loops: 8
 regression_6000.c:1784: xtest_teec_open_session(&arg[m].session, &storage_ta_uuid, ((void *)0), &orig) has an unexpected value: 0xffff000c = TEEC_ERROR_OUT_OF_MEMORY, expected 0x0 = TEEC_SUCCESS

Use 3 threads instead of 4 to reduce the memory footprint.

Link: [1] OP-TEE/optee_os@cda03b6
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Etienne Carriere <etienne.carriere@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>

host/xtest/regression_6000.c

@@ -1770,7 +1770,7 @@ static void *test_6016_thread(void *arg)
 }
 
 
-#define NUM_THREADS 4
+#define NUM_THREADS 3
 static void xtest_tee_test_6016_loop(ADBG_Case_t *c, uint32_t storage_id)
 {
 	struct test_6016_thread_arg arg[NUM_THREADS] = { };