test/py: avb: fix test_avb_persistent_values fail

Fix test_avb_persistent_values() pytest, which was failing because of
wrong size value provided from tee sandbox driver.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
diff --git a/drivers/tee/sandbox.c b/drivers/tee/sandbox.c
index a136bc9..2f3355c 100644
--- a/drivers/tee/sandbox.c
+++ b/drivers/tee/sandbox.c
@@ -178,7 +178,7 @@
 		if (!ep)
 			return TEE_ERROR_ITEM_NOT_FOUND;
 
-		value_sz = strlen(ep->data);
+		value_sz = strlen(ep->data) + 1;
 		memcpy(value, ep->data, value_sz);
 
 		return TEE_SUCCESS;