Commit 98681675 added a mechanism to track statistics corresponding to
the spilling of changes from ReorderBuffer but didn't add any tests.

Author: Amit Kapila and Sawada Masahiko
Discussion: https://postgr.es/m/CA+fd4k5_pPAYRTDrO2PbtTOe0eHQpBvuqmCr8ic39uTNmR49Eg@mail.gmail.com