database/sql: add test for Conn.Validator interface

This addresses comments made by Russ after
https://golang.org/cl/174122 was merged. It addes a test
for the connection validator and renames the interface to just
"Validator".

Change-Id: Iea53e9b250c9be2e86e9b75906e7353e26437c5c
Reviewed-on: https://go-review.googlesource.com/c/go/+/223963


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>