database/sql: make WAIT tests more robust, rely on waiter trigger

Replace the WAIT query prefix with a function callback.
This fixes timing issues when the testing on loaded servers.

Fixes #51208

Change-Id: I5151b397b7066c27ce6bc02c160dde0b584934bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/385934


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Trust: Daniel Theophanes <kardianos@gmail.com>