Before closing up the wall, it is absolutely essential to test for leaks. Use a test plug or a tightly packed rag to block the drain from inside the tub. Fill the tub with 3-4 inches of water. From the access panel, use a flashlight to carefully inspect every joint you made: the drain shoe, the overflow gasket, and all cemented connections. Look for even the smallest drips. After 15 minutes, if there are no leaks, remove the plug and let the tub drain completely. As it drains, watch the connection at the P-trap to ensure it is also watertight. If you find a leak, you must drain the tub and fix the connection.